Highly effective & inexpensive tool for detecting bad bills. Simply make a small mark on U.S. currency. Chemically sensitive ink works instantly. An amber mark passes the test, while a dark brown or black mark means the bill is probably counterfeit. Works on all new design U.S. currency (bills series 1959 & later). Counterfeit Bill Detector Type: Pen; Marking Color: Amber; Counterfeit Indication Mark: Black; Dark Brown; Currency: U.S.