Go by the serial number of the LP. Example:
Elvis Presley (1st album) LSP-1254
That is the serial number of the LP. If it has a (e) after it then it is a reissue. All original LPs has no (e) after the number. Also, go by the LP label itself too.
Example: from 1954 to 1968 RCa logo looked like this:
Other reissues labels with go by the year they was released in.
Example: If it has a label similar but doesn't look just like the original then it is from the 60s. If the label is orange then it is from 1968 to 1974. Tan is 1974 to 1975. Then a redone of the label with Nipper is from 1976 to 1977.
The most commom is the ones from 1977. That is when every Elvis record that he did from 1956 to 1975 was redone to match up to the label as seen on Moody Blue, Welcome To My World and From Elvis Presley BLVD.
The photo I provided is the original LP label for Elvis' 1st album. I see that there is a few on there right now.
Also, look on the lower left hand corner. If it has either RE or SER then it is a reisssue.
Here is auctions for the original 1956 LP: