The Dri Mark Counterfeit Detector Pen DRI3513B is ideal for use in busy retail environments to help you identify potentially fake bills. As customers are checking out at the cashier, this quick and inexpensive currency-screening device can be used in conjunction with other counterfeit detection methods. It's easy to use; all you need to do is mark a small line on U.S. currency. When the mark is a light amber color, the bill is genuine. If the ink is black or dark brown, the bill is suspect. This counterfeit money detector pen has enough liquid in its barrel to mark dozens of bills. Stock up on plenty of these three-packs for each cash register, so you'll always have plenty on hand. The Dri Mark pen is also available in additional quantities. There are additional counterfeit-detecting accessories in the product line. All items are sold separately.