Drimark Counterfeit Detector Pen, 3 Pack (351RET3B) Make a small mark on US paper money. If the mark stays Yellow or clear, the bill passes the test. If the mark turns black, the bill may be counterfeit. STOP loss due to counterfeit Now in a convenient retractable! no more lost caps and dried out pens. 3 pens per pack Easy and quick to use. The most effective test for US currency paper. Dri mark is the original and best counterfeit detector pen