I recently purchased a Picasso lithograph from a reputable art dealer. I understand how lithographs are made, but how can one tell if something is an original lithograph or a copy of a lithograph? Is there an easy way to tell the difference?

 The answer is more than a yes or no. I would like to approach the answer to your question assuming you don't have much information about the difference between hand lithograph and offset lithography. This way I can give a more complete answer.

When a hand lithograph is made the artist draws directly on the printing element. (stone, aluminum, Mylar, etc.) From this drawing the prints are inked and pulled. One drawing for each different color. Each print records the unique mark of the artist and their hand.

When prints are made using offset lithography, an original (painting, drawing, watercolor etc.) is taken to a commercial printer. The printer photographs the original and then converts all the colors into a combination of 4. (Red, Yellow, Blue, and Black) A negative is made for each of these colors and a photographic plate is prepared for printing. They are run, usually all at once on large high speed presses. The mark of the artist is lost in the translation from original to photograph and then
color separation. Depending on the printer, colors can vary drastically from the original.

A common way to tell if a print is a hand lithograph or an offset lithograph is to look at the print under magnification. Marks from a hand lithograph will show a random dot pattern created by the tooth of the surface drawn on. Inks may lay directly on top of others and it will have a very rich look. Prints from an offset press will show a mechanical dot pattern from the color separations. Each color generates a separate dot pattern that when combined produce a very small circle or "rosette". The dot patterns look like the dot from the newspaper comics but smaller. You can easily see these dots under small magnification or sometimes with the naked eye. All the dots line up in neat little rows. If you can see these dots it is a sure sign of camera and commercial involvement. If the dots are random and you purchased the print from a reputable dealer it is most likely a hand drawn lithograph.