Geez just invest a few dollars in some of those counterfeit detection pens. Problem solved.
Was talking to my neighbor a couple months ago (she works at the local police station a few blocks away). She knows about all the calls the police respond to. They've been getting reports of counterfeit bills from a couple stores where they're holding a perp. Police arrive, and surprise, the bills are genuine. They even confirm with a local bank. Turns out some of these counterfeit detection pens only work with new bills, and not older bills in circulation. So no, having the pens does not solve the problem. (I've since started converting older bills from my stash under the mattress to newer bills.)