There are many different, and inexpensive counterfeit exposing devices. UV flashlights ($12) are recommended by the SS.    https://www.thebalancesmb.com/best-counterfeit-money-detectors-4160584  Here's an article that explains how to spot a phony bill under UV (color changes). https://6abc.com/news/counterfeit-bill-makers-find-way-to-avoid-detection/2059956/