In Basel, Swiss authorities are prosecuting a local art expert who they say sold hundreds of fake prints that he passed off online as the work of Roy Lichtenstein, Andy Warhol, Paul Klee, Pablo Picasso and others over 10 years.

In New York, Adrienne R. Fields now spends much of her workweek scanning the internet for forged prints that pop up at website after website. She is head of the legal department for the Artists Rights Society, which protects the intellectual property rights of artists and their estates.