There is no sure-fire technique for verifying a record's first pressing, but there are some basic tried-and-true methods that will get the best results. They require information on how vinyl LP's are made and distributed and an exhaustive knowledge of music in general. Knowing how LP's are printed and distributed can make the difference between paying far too much for a second or third pressing of a recording or selling a very valuable first pressing for far too little. Many record collectors think they own a first pressing when they don't, and many novice record collectors don't realise when they do own a first pressing.
Look on the spine of the LP record sleeve. First pressings will usually have a four letter/four number combination, such as ABCD-1234. And anything after a second or third pressing will have a two letter/five number combination, such as AB-12345.
Go to Discogs.com or Recordgeek.com to verify your first pressing. You can do this by matching your catalogue number to the catalogue number on Discogs.com. Or you can talk to folks on the Recordgeek.com forum and match your record up with theirs.
Use the latest edition of the Goldmine Record Album Price Guide as a third reference. You can find most any LP in this book, from the most popular pop records of the 50s and 60s to the most obscure art-rock records of the late 70s.
For older, more obscure international reggae and jazz LP's, it is almost impossible to find out for certain whether the record is an original pressing. Older reggae albums were not widely distributed and reggae record labels had varying methods of cataloguing that many times did not include the year of release. When verifying the pressings of your record collection, match all the information on the sleeve with the original pressing. Luckily, websites like Discogs.com make this process a lot more simple than it used to be. You will need to match the catalogue number, year of release, exact artwork, colours and liner notes, if available.
Don't assume that because an LP is shrink wrapped it is the original pressing. This is an old trick by many less-than-forthright record sellers to get buyers to think the record is the original. Don't fall for it.