Counterfeit Bill Detector Marker Pen, 3 x 7.25 x 0.6, Black, 50/PackBusinesses 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.