Forget about checking bills’ authenticity by exposing them to light. Reduce wait time at the cash register and increase peace of mind with one of Dot It’s counterfeit detector pens. Our counterfeit detector pens reveal counterfeit U.S. currency with a single, quick test. Ask your cashier to make a small mark on any paper money: if the mark turns amber, the currency is authentic. If the mark turns black, you know it’s a counterfeit.