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.
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