Carry a pro kit.
Credit: Martin Hospach / Getty Images

Professional authenticators put together a fake-busting toolbox, and so can you. First, you'll need a loupe (those single-eye magnifying glasses used by jewelers). "You can take a closer look at signatures, chips, all that kind of thing," says Dowling. You should also carry a tape measure and a color chart that has information about historic pigments. "You could be looking at a painting that's [supposedly] 200 years old and all of a sudden find [a certain kind of] yellow that wasn't available until 60 years ago. Your little color chart is going to stop you getting caught out." Finally, pick up a cheap black light. "If you wave it over glass, you can see where it's been mended," says Dowling. "If you wave it over a painting, you can see where new paint has been added." And the tools aren't just useful, they also send a message. "If you just have those few tools like these in your pocket, two things are going to happen," he says. "You'll be able to identify damage yourself, and when you pull out your kit, that seller is going to take you a lot more seriously."