Take a look at the print closely. If it's an original, you will know rather quickly because lithographs are made in only two ways: by an artist or a machine. Prints made by machines, otherwise known as offset lithographs, can easily resemble prints pulled by artists. Offset lithography is a high-speed commercial printing process. Such prints are not originals. So your goal is to rule out the use of an offset lithograph printer. As you do, you can easily discover if your suspected original is an offset lithograph (of no great value).
Check the print for an edition number or the letters "A.P." If a number or the letters "A.P." are visible, you have a suspected fine art print. Fine art prints are either numbered or branded as artist proofs.
Secure a pocket microscope or magnifying glass.
Prepare the work for inspection. You may need to remove the work from its frame.
Inspect the artist's mark or the numbering. If either appears sharp, continue inspecting.
Look for evidence of a mechanical dot pattern.
Examine the work for any dots lining up in neat rows.
Rule out the presence of "rosettes." Rosettes resemble dot patterns from old newspaper comics, only they will be smaller.
Conclude that if you see evidence of mechanical production, such as rosettes and neatly lined-up rows of dots, you're inspecting an offset lithograph and not an original.
Assume that if you see random dot patterns, or no dots at all, you have an original lithograph---especially if you see irregularities in how the ink rests on the paper.
You may be able to discover the "rosette" pattern without a magnifying glass or pocket microscope, depending on how good your vision happens to be. If you doubt what you see, get a second or third opinion before forming a conclusion about the lithograph.
Handle potentially valuable fine art prints with care.