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
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 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.