As a banker, I know that the best tool for detecting fake bills is... the human fingers. Between the top-secret rag paper and the Intaglio printing process, US bills have a very distinctive feel to them that your casual counterfeiter can't duplicate. You hear it over, and over, "It just didn't feel right." There are verifier pens, blacklights, and what-not, to find out if one is really fake, but it's the feel that usually sets off the mental alarms in the first place.

When I was a kid, you could run dollar bills through a copier and they would be good enough to fool the change machine down at the car wash. Back then, and 8-year-old kid with $10 in stolen quarters in his pocket was practically Al Capone. Allegedly, of course. I never did anything like that. Ahem.