Look for the signs of an artist's print. For example, most prints feature the artist's signature written in pencil, as well as two numbers separated by a slash (the bottom number tells the number of prints made off the plate in total; the upper number signals the print's individual number within the series).
Learn how lithographs are made; this will help you distinguish lithographs from other type of prints (such as etchings or woodcuts). Artists make lithographs by using a special type of waxy crayon to draw on a flat stone; after a series of chemical processes, ink sticks to the areas covered by waxy crayon and not the rest of the stone. Because of this, lithographs can look like drawings. For an example, see the famous lithographs of Käthe Kollwitz.
Do your research. Artists often play considerably with media, so the best indication of whether a work is a lithograph or another type of print is the artwork's provenance (where it came from). Reputable galleries will include the artwork's material, so if you look at wall text or a gallery list, you should see what kind of print you're looking at.