lieutenant mike brotherton demonstrates how those detector pens work. 42:09-:13 "on this bill here, if i mark it, it turns brown." that's one of the counterfeit bills they've found. take the marker to a real bill, and the mark shows up yellow. 39:45-:51 "most of the ones we have received, they're going with the same serial numbers, so we can compare that."brotherton says businesses can also buy black light machines that reveal hidden stripes only found on real currency. he says either way, it's a good idea for business owners and managers to educate their employees, as accepting a fake 100 dollar bill can lead to a significant loss. 40:16-:21 "once you turn it into the bank, the bank checks the $100 bills, and they will take it out of circulation and you're just out the money." way to check it yourself, but you're still not sure, to call police. he says investigators from cedar rapids and go over security footage from stores where the bills have been used. he believes this counterfeiting operation is not based in iowa city or even eastern iowa, but likely out of state in a bigger city.