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
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?