The paper money you’re holding should change color. “One of the easiest ways ... There are also counterfeit pens, often coming in a pack of 5 for $10, that purport to find fake bills.

If it does not, it is most like a counterfeit note. There are a lot of subtle details in a genuine banknote. Color-shifting ink is a great indicator of whether money is real or fake. For example ...

The operation saw the production of fake sterling and euro notes that, police said, were good enough to beat the counterfeit pen test ... produce the counterfeit money. The cash was then available ...