Hunting for 1st pressings

How do I know if I'm looking at a first pressing of vinyl? 
Also, I've seen ads for new, sealed vinyl claiming it's a 1st pressing. How does the seller know that if the album is sealed. 

There's generally no way to tell if a sealed LP is a 1st pressing,  because catalog numbers stay the same for several runs. Disgogs is the only source I am aware of, and it's never easy. In other words, I've seen matrix #'s of early pressings not identified on Discogs. I love first pressings, but identifying them is a real pain..... It's almost as if they thought we wouldn't care. If they thought we would care, they would have made it easier.

Easy to identify with Japanese pressings from 70s. I hunt for Japanese first pressings pro - very rare and quite expensive.
I never pay much for sealed records. Even if they are not re-sealed, you never know what's in there and the condition. Sometimes vinyl deteriorates substantially with age. When I say I don't pay much I mean over $100.
Much of it is anecdotal; at the time the records were made, consumers weren’t really focused on first pressings as such. Now, trying to collect older pressings, you need to rely on a combination of indicia, not all of which is described in one place. That includes the deadwax inscriptions, covers, labels, inners and record plant markings. First pressing from what country? And don’t assume that a first pressing is necessarily the best sounding.
Sometimes, it’s a fluke. For example, I found that a later MCA pressing of Lynyrd Skynyrd’s first album sounds better than my early SOS label (or the MoFi reissue for that matter). Also, on Free’s Tons of Sobs, it is commonly assumed that A2/B2 is a first press, but I’ve seen A1/B1’s offered, and sometimes, those aren’t on the earliest labels- the Island pink label "orange bullseye."
Go figure.
For some US records, multiple lacquers are cut at the same time, to provide numerous pressing plants with sufficient metal parts to press large numbers of records. Thus some indicia as a 1, 2,3 in the deadwax may not be indicative of earlier or later, but are essentially simultaneous. 

Thanks for the responses. Apparently there's no simply way to identify a 1st pressing. Therefore, I won't pursue them. 

Some can be identified by cover details. There are may be notes only belonging to the first edition. Clear example is Magical Mystery Tour where later editions had many faces blanked out because public figures did not want to be displayed on the record cover. Same story with Rolling Stones "Some Girls". Another clear example is Led Zeppelin "In Through The Out Door" with bunch of different cover variations.
Many shaded dog releases would mention on the cover "previously released as (on different serial number)". 
There are also labels that only did re-issues such as Arkiv re-issued Deutsche Gramophone
Apparently there are and there aren't simple ways to ID 1st pressings.
It really depends on the record- there are some 'known' first pressings that have a history that has been documented. Are you looking at rock/pop stuff or classical/jazz? Very different markets with different knowledge bases.....
First pressings are often if not always best sounding and worth pursuing.
+1 on last comment by inna. Worth the effort IMO.
As an example. I have Pangaea by Miles Davis on Japanese vinyl, three pressings - original pro, original regular and same year reissue.
There is no doubt that the original regular is better sounding than reissue, and the pro is better still and not by little.
Another interesting example comparing original American and original Japanese. Bitches Brew by Miles Davis. They sound somewhat different - the American is a little more raw and edgy and Japanese is quieter and more refined, both are quite dynamic. I listen to both American and Japanese, though Japanese more often.
And the last example. Inner Mounting Flame by Mahavishnu Orchestra. Original Japanese and Japanese first reissue. They sound different, the original is a little more dynamic and the reissue is more refined and a bit laidback. Both great, I listen to the original.
I agree with inna, first pressinigs almost always sound better. I have even come across a few 'well used' first pressings that are astonishingly good after a cleaning.  I almost always seek them out. In my collection, the first pressings are always more dynamic then later ones, especially most reissues..

Discogs is pretty helpgul for this sort of thing.
It's always good to have all possible pressings of a same thingie so you can compare and decide. 3 pressings good 5 pressings better. good luck.
A good dealer will do alot of  the work for you! 

A trick I use is if I see it is a record club pressing it is not a first pressing. If you see Crc,Rca or Bmg record club do not buy.
Club pressings started in early or mid-80's. Not all of them suck. Came across good and terrible ones.
As an example, if anyone will have a first experience of RCA club pressings of Ziggy Stardust, one will never again would want to listen to David Bowie -- It's THAT terrible. Most of new wave club pressings are OK.
CZ- did you mean '80s - I remember record club offerings in the '60s and early '70s. I think your statement earlier, about listening to multiple pressings, is the only way- in some cases, the first pressing sounds better to me, in other cases it doesn't, at least with rock/pop stuff.  Sometimes, it is a question of preference -detail v. a more organic quality. Each different pressing has strengths and weaknesses in my experience (apart from the downright awful).  And I think the question remains, "first" from which country?