Counterfeiting currency has been going on for years, and is, as PBS puts it, as old as money itself. And in a recent WCCO-TV report the tactics that we have used in the past to determine if a bill is fake or not might cut it anymore. 

Right now in the Twin Cities area, it looks as if there has been a rash of fake $100 bills being passed around, and in some cases, the pen people used to mark a bill to determine if it was fake, aren't cutting it anymore as counterfeiters are becoming more sophisticated in their methods.

Experts say in lieu of the pen, businesses should look to invest in a UV penlight. The UV detector works by verifying a UV mark made on the bill by shining ultraviolet light onto the bill. If the UV printed image/mark glow when subjected to the UV light, then the banknote is expected to be authentic.

So which bills should you be marking up? The Federal Reserve in San Francisco states that the $20 bill is the most commonly counterfeited banknote in the U.S., while overseas counterfeiters are more likely to make fake $100 bills. The real $100 bills are more prevalent overseas as well.