Lithographs are copies of artwork that are either created by the artist or an authorised maker. Greasy crayons are used to make a mirror image of the work on a stone, and then paper can be pressed onto the stone, creating a replica of the image. Prints however, are usually mass produced, and are typically done by mechanical devices of some type. They're not the same as a poster that you get from the store, but they aren't as close to the real deal as a lithograph.
Ask the dealer from whom you're purchasing it if they're aware of whether it's a print or lithograph. If they claim it's an original lithograph, ask for certification. If you're not buying it from a dealer, examine it yourself.
Put on your gloves and remove the artwork from its frame. Make sure not to touch it with your bare hands. Make sure the ink is not raised from the page either, which would indicate that the work is an etching rather than a print or lithograph.
Look for a signature and a number. Whether the piece is a limited edition print or a lithograph, there should be a signature by the artist with a number and a slash. For instance, if you had the 13th print of a limited edition set of 50, you should see the artist's signature and the symbol "13/50." If your piece hasn't got these things, it's probably not an "original."
Use a magnifying glass to closely examine the artwork. Pay attention to the dot patterns. If the dots are in tightly unified rows, this is an indication that it's a print. If the dots are more randomly scattered throughout the page, it's an indication of an original lithograph.
Look for any signs of mechanical reproduction, including ink patterns that are too neat, a lack of apparent brush strokes or anything else that appears to have been done by a machine. A print will more than likely show indications of mechanical involvement.
Ask for input from someone who's well versed in art to help you assess the work if you're still unable to determine what it is.
Don't touch the work of art if it doesn't belong to you or you're afraid of damaging it.