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. 
rockyboy
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.