Businesses have counted on the Dri Mark Counterfeit Money Detector Pen for over 25 years - you can too! An efficient option for loss prevention related to counterfeit U.S. currency. These are easy to use - just put a small mark on the bill being presented to you. If the ink remains yellow, you know this is real U.S. tender; if it turns black you immediately know not to take the bill. It is that easy and super fast.