Smart Money Counterfeit Bill Detector Pen for Use w/U.S. Currency, Dozen, Sold as 2 Dozen Counterfeit bill detector pen for use with U.S. currency. Simply make a small mark on U.S. currency. Chemically sensitive ink works instantly. Amber mark passes the test, while a dark brown or black mark means the bill is suspect. Works on all new design U.S. currency (1959 and later).