The "deadwax" on a record - the space between the end of the program and the label, where the runout groove exists, contains a matrix number and sometimes other information. You can read the matrix or other information there. Sometimes there are stamped items - denoting the mastering lab - MASTERED BY CAPITOL, STERLING, etc.
If there's a signature by "Wally" or "Porky" it's gonna sound fantastic - there are other initials and signatures to look out for from the great mastering engineers.
Usually the first pressing will be identified by a "1". The Who's album "Who's Next" has a famous first pressing which is reputed to be the best sounding version of the album - look for "W1" on the labels. Sometimes the first pressing is noted by a "2" or even "3" or later - if the first master was flawed and they had to make another one.
It's really on a case-by-case basis for each record. Obviously the first thing to look for is the label. Knowing the history of the label is key. If that "Long Tall Sally" EP has the old "Rainbow" 60s label, that makes it more desirable. If it has one of the many other later Capitol labels (including the confusing 80s "Rainbow" nostalgia label) or the Apple label - not so much.
Back to the OP: Even if you decide not to keep these, do not drop them off at a Salvation Army or other thrift and don't throw them out. PM me, I'll buy the lot today. Or bring them to a local used record store or a local record show. Even if you only get $2 each, as I originally wrote, stuff from Stones, Beatles, U2, The Who, Pink Floyd, Led Zeppelin, and other Big Artists are worth something. Little known acts - not so much. Ultra-oversaturated acts (Billy Joel, Barbara Streisand, Eagles, Doobie Brothers, etc.) not so much.
If you go to a used record store they'll probably give you something like $1-2 each for the Beatles, Stones, etc. records (unless they are in absolute mint conditions), and you'll be lucky to get 20 cents for the more common stuff.
