(PORT HURON TOWNSHIP) -- The St. Clair County Sheriff's office is warning the public of a counterfeit bill scam that has hit St. Clair County. Sheriff Tim Donnellon says that businesses in Fort Gratiot Township, Port Huron Township, and in the city of Port Huron have seen a rash of counterfeit $10 and $20 bills in the past two months. The counterfeit money is being passed at gas stations, fast food restaurants and pharmacies. Donnellon says the scammers are also buying items from Craigslist and using counterfeit money. A couple of tips on spotting a fake bill include holding a $10 bill up to the light. You should see a hologram of Alexander Hamilton on the front right hand side, just above the number 10. There will also be a hologram that reads: U.S. Ten on the upper right hand side of the bill. On a real $20 bill, there is a hologram of President Jackson on the front right side above the number 20. There is also a hologram reading: U.S. Twenty on the front left hand side. Counterfeit bills will not have these markings. If you have received a counterfeit bill or have information on the scam operation, contact the St. Clair County Sheriff's Office.