Examine the print for obvious signs that it's a reproduction. Currier & Ives Original Prints suggests looking for such words as "reproduced," "from the collection of," "courtesy of" or "printed in the USA," as well as copyright symbols from book or art publishers.
Look for "chops" -- embossed symbols unique to a print shop and printer.
Request documentation showing the authenticity of a print that doesn't have chops; originals printed by the artist may not have them, but the vendor may have a certificate of authenticity that proves its originality.
Use a magnifying glass or loupe to get a very close-up view of the ink. Look for the tell-tale signs of an original lithograph -- an image made up of randomly spaced dots or of solid, albeit often uneven, tones. Assume that images made of evenly spaced dots are offset reproductions.
Search for such signs of original lithographs as pencil signatures and series markings showing what number print a particular lithograph is in a series, but don't accept their existence as absolute proof a lithograph is an original. Marks can be reproduced. Weigh them against other signs of originality or reproduction.