How do you know when an LP is a first or early pressing?

Being relatively new to analog, there has been a steep learning curve.
The latest important bit of knowledge has pointed me to the codes stamped in the runout groves, the labels and the sale listings. The questions are how to read the codes, and what to look for on the label and on the sale listings? For instance, a friend guided to to the “pink label” British Island Records pressings. Believe these are all first pressings and the original British. I bought a couple including Emerson, Lake and Palmer’s first album. A pretty good used one cost me $25 which I understand is a good price. I would like to find and buy more of these early, or first, pressings. I understand their sound quality is normally, or frequently, excellent.
Here we go.... While there definitely is a big grain of truth to that, the bigger truth with vinyl is you just never know. There is a strong tendency for original early pressings to be better, but its really more like its the later and reissue pressings that are worse. In other words buying new original is no guarantee of exceptional sound quality.

If all you want to do is avoid the really bad stuff then you are on the right track. But if what you really want is the exceptionally good stuff then it is just the beginning. Because if you buy 20 identical copies and play them you will find no two sound exactly the same. Most will be very close (average), a few will have pretty obvious problems (sibilance, weak bass, etch, etc), and may be one will sound quite a bit better than any of the others.

I have records just like this. Never realized it until I started buying and compared their Hot Stampers with my identical copies. They look exactly the same, identical hot wax and everything, but they do not sound anything like the same. Hot Stampers are in a league of their own.

So you can filter like you’re talking. That all by itself is better than most ever figure out. Then you can compare copies and keep the best. Knowing its all down to random chance and you might do this with 10 or 20 or more to find a really good one. Or if you really just want the to die for sound and its worth it to you then buy a White Hot Stamper. Only then will you know what I’m talking about.

Then if you do please come back and let us know, as there are still a lot of skeptics who think I’m blowing smoke.
Discogs is your friend

It's not 100% accurate for listing EVERY pressing, but still the best resource,so far.
SQ varies with every copy.

You can save some time here, but it you will pay for it.

The site is down. If they're left coast the inventory could be melted.
mglik, it can go either way. Although the masters may be better back in time popular records often got cut short in pressing quality. A good master with top notch modern pressing technique and premium vinyl can yield a superior result. This does not take into account that most popular records were not taken the best care of. I personally will not buy used vinyl. I have thousands of old records. I do not need any more. Some reprints are actually far superior to their original versions because the original masters were terrible. A good example of this is the early Roxy Music records. The 1/2 speed remastered versions are worlds better than the originals. Some of Frank Zappa's early catalog were mastered on second rate systems. Before he passed he remastered several of them with exceptional results. Far superior to the originals. 
The site is down. If they're left coast the inventory could be melted.
+1 Discogs. Great resource.

I’ve cataloged all my physical media on Discogs. Takes much time (depending on how many you have) but worth it.

But also agree that not all first pressing are the best, even though they might be worth more in some cases. Many times a reissue/remaster can sound better. It just depends.
On the subject of reissue/remaster, I have found that the best reissue/remaster copies sound phenomenal when the right people did the mastering, and the pressing and vinyl quality are first rate.  You have to research to find who did it and who pressed it. I have also been very disappointed with some reissues, particularly those that are sourced from digital files.  The Pink Floyd reissues, on Pink Floyd Records, are fine examples of excellent mastering and pressing.  Impex does a phenomenal job, I highly recommend the Jennifer Warnes latest reissue of Famous Blue Raincoat and produced by Impex.  It's stunning!  The list goes on, but I believe that you get the idea.  Originals, as others have said, can be a crap shoot.  Unfortunately that is indeed the case with originals, just as it is with reissue/remasters.  
So, Really this just comes down to treasure hunting? I have been buying vinyl for the last 10 years. Great sounding vinyl can come from a forty year old Verve record that you found in a used record store, or a fantastic sounding copy of Steely Dan's Aja on ABC records at the Antique mall, or a very good sounding new copy of Eric Clapton greatest hits from RTI. The Austin record convention is a fantastic place to find rare things and of course its all hit or miss!

Matt M
ELP’s first album is one of the last pink labels before Island started using the pink rim in the UK. By the time of the pink rim, Island was pressing and distributing through EMI and the deadwax reflects EMI nomenclature. Previously, Island worked with Polygram, which used a different type of code in the dead wax. (Every once in a while you’ll find an old pink rim that used the metal parts from an earlier era-- I have a Free Tons of Sobs that bears the Polygram nomenclature even though by the time of that reissue its manufacture should have been EMI). The early pink labels are yet again different, some pressing at Orlake, with different nomenclature--Those tend to be the "bullseye" pink labels. I suppose one might find an Orlake on a slightly later pink label- after bullseye, there was the ’blocked I" in black, then the white unblocked "i," which was the last variation. These would be shown in a book like Yuri Grishin’s (if you are referring to his series on famous British labels). At the time this stuff was manufactured, it was a commodity and the average user wasn't necessarily focused on pressing plants. Even the record company executives wouldn't have paid much attention to pressing plant issues, except for business reasons; the relationship between deadwax and sonics was not even on their radar. Much of this information was unlocked by collectors years after the fact. Island was sold to Polygram and eventually merged into the Universal empire. Who knows what documentation exists for manufacture- it wasn't necessary paperwork that would have carried over through various company transfers. 
And that’s just the pressing plant! Island also used a variety of mastering engineers including Sterling in the U.S. and George "Porky" Peckham, whose work (he often did the remasters in the period) was bombastic.
I generally prefer the earliest pressings on Island for sonics, e.g. Orlake over Polygram and Polygram over EMI- this is reflected in the prices as well-- The EMIs tend to sound a little hotter and brighter, the Orlakes, though a little noisier, are richer sounding. Some of the early pink labels fetch real money these days. Chris Blackwell and his crew were way ahead of the pack in signing very forward looking artists and wound up with a roster of talent that was the envy of the majors. Island essentially forced most of those labels to start signing more adventurous acts; thus, the imprints from the majors with more psych or prog sounds, e.g. Philips formed Vertigo, etc. Lots of great stuff on Island from the era.
Took me 20 years to figure this out and Im still learning.
It was actually Ralph who turned me on to these issues. He told me that he regularly uses a “pink label” ELO first pressing at shows.
He also told me to pay attention to the codes stamped into the run out groves. That they list the early or first pressings. I think that there is a indication by the number “1” within these stamps. But I don’t exactly understand where to look and exactly what to look for.
Charles, I did get a couple of Better-Records Hot Stampers. First I got a Super Hot Stamper. True, the vocal was clean an nice but the accompaniment was as usual. I was not impressed and sent it back.
Thinking that it must be the lower level Super and not White, I got a White Hot Stamper. A symphonic work, it was so offensive, I could not listen to the B side! I also returned it. Now I have cold feet and will not spend another $199+ again. I do imagine that your experiences are very “White Hot”. It is a lot of money for 40 minutes of hot sound. I have a lot of old, so so, recordings but find that I usually can enjoy the performance and live with the sound quality. I do find that I most enjoy the 180 gram remasters. 
Just use for every record if you want to know how many pressings made, first in the list is 1st press
A good example of this is the early Roxy Music records. The 1/2 speed remastered versions are worlds better than the originals.
I go with the country in which the recording was made. Roxy being a British band you really want the Brit originals.

The more transparent your system is the easier it is to hear the differences. Resolution is important- stuff that sounds distorted on lessor systems can simply be detail- that a proper system will bring out and reproduce instead of getting freaked out about it. A great example of this to me is on Nonsuch 'Bulgarian Village Music' which is a field recording and has a bit of power in the vocal regions... I've heard many systems freak out on that LP, but in fact its immaculately recorded and stunning on a highly resolved system.

Its a real treat when you find a gem and play it on a good system!
As Millercarbon said, the hot stampers from better records are great! One example of an original not sounding as good as a later reissue Is Miles Davis Kind of Blue. The original 6 eye Columbia is inferior in sound quality to the very early 70’s reissue on the Columbia red label.
OP, I can give you some examples what the numbers etched into the dead wax mean. Each record label has their own system.

Here’s an early 1st pressing of a Kate Bush album, the matrix number is:
YAX 5389-1
YAX 5390-1
The catalogue number and county of origin is included;
89 is side A, 90 is side B. The -1 means it’s the first production run on stamper #1.
The question is was this vinyl pressed early in the run when the stamper was new, or was it pressed late when the stamper was becoming worn and needed to be changed?
The only way I know of is to play it. There may be additional etched numbers which identify it. And the mastering engineer and record plant code may be scratched into the runout. A -2 may also be on a first pressing.

YAX 5389-4
YAX 5390-5
These are pressed later with a different stamper but may be of better SQ because they were at the beginning of the production run.

As stated earlier, the label is important in identifying early and late pressings. Record companies change the label for various reasons, eg, which country it’s released, reissue, early issue.

Steve Hoffman forum has lots of info on first issues and best quality pressings.

Discogs usually has all the info you need. The real fun starts when your buying records that Discogs doesn’t know about (yet)!
Mglik, I think that you are correct to be wary of ’hot stampers’. To my mind, the history of a record is a lot more important than its stamper: you know, what kind of blunt stylus was used to play it, if it was used as a dog’s frisby, and so on.

Let me tell you about something that you can control: how clean it is. Once you get into ultra-sonic record cleaning, you’ll never want to go back.

Good luck!
Discogs usually has all the info you need. The real fun starts when your buying records that Discogs doesn’t know about (yet)!

Then you can upload all the info yourself.
If there is not info yet then it will be there in the future, even if the record is extremely rare. The progression of discogs is huge, I remember when it was useless source, now it’s the best place to find all info about records, everyone can add or change info (if there is an error). 
Some speak so highly of original pressing SQ. As others have stated this is a quite variable topic that cannot be black and white. What can be is each pressing’s value. If this is important to you then the original pressings often have the highest value and you hope better SQ as well. There are often reissues that have excellent or better SQ, like MoFi and QRP. It takes research and simply buying multiple versions and let your ears decide.

I have not yet taken the leap for a hot stamper, but with the 30-day guarantee, I will likely do this soon. It really makes me wonder how many I have in my collection. My original copy of Bruce Tunnel of Love sounds amazing so I like to think of it as a Hot Stamper : )

You must not be on Discogs so that is definitely your first step. This is not only to know all versions released but to catalog your collection.
There are two distinct inquiries here- one is how to determine if a record is truly a first pressing, which not only involves an examination of deadwax nomenclature, but label, including logo, sometimes the packaging, and what country of issue as well. There’s a lot of minutae in this but the info is out there on various websites for different labels, bands and genres and Discogs is a good place to start.
The second area of inquiry is connecting this information with sound quality and that’s more difficult in some ways because it depends on listening evaluations and comparisons. You can do this yourself if you have the time, money and ability to source multiple issues. You can rely on anecdotal comments from others who have done the comparisons. For classic rock, the older threads on the Hoffman board are good, at least those that are in depth and not simply comments stating that their copy "sounds good." The Better Records/Tom Port thing is a whole other subject that is fairly controversial and I’m not going to wade into that.
In some cases, with rare jazz records, there aren’t numerous pressings- often just an original run and sometimes a second pressing, followed years later by reissues, often with a big question mark about source- needle drop, digitized, etc.
I have found, in the classic rock area, that the sonic differences do not necessarily correspond to any rational relationship with when the record was issued-- oddball reissues by the label itself that sound better than first pressings, differences in EQ and tonal balance that may favor a later (but still early) issue over a later one. There are also "known" hot pressings of some of these records. I think it is very hard to generalize and the knowledge that has developed over the years, after the fact, by collectors and sound hounds (two different species) is often siloed by band, particular album, or label. There are also shortcuts in some instances that can save you money where the metal parts from an early desirable pressing show up on a later, much cheaper period reissue. The quality of the vinyl used at the time is also a factor, so there are many variables. I have found that I have accumulated many copies of records I like but aren’t the best recordings or pressings and the end result is comparing different sets of compromises, occasionally resulting in no one "best" sounding copy, but shadings of difference with various strengths and weaknesses.
This is, as EBM said, something that you develop over the years, with accumulated knowledge based on first hand experience having gone through a lot of records. I have done purges over the years to get rid of thousands of records that didn’t make the cut and got sidelined-- to the point where I needed to make space and saw little need to keep the lesser copies. That said, I still have some albums where I probably have a dozen different copies. This was not the result of just a few years, but decades of buying.