Dri-Mark Counterfeit Detector Pen - Patented pen reacts to wood-based copy paper. Why take a chance on accepting large counterfeit bills? MMF Industries patented Counterfeit Detector Pens are easy to use and require no training. The iodine in the pen reacts to wood-based copy paper leaving a dark brown or black mark; the pen will leave an amber mark on fiber-based paper used in authentic currency. For best results, use in conjunction with other counterfeit detection methods.