Authorities are warning: Watch out for counterfeit bills...This after a local business loses out. Erbert's and Gerbert's in downtown Mankato accepted a fake100 dollar bill last night. The sandwich shop didn't realize the bill was fake until it made its bank deposit. Authorities say they've seen mostly twenty dollar counterfeit bills... and they're warning businesses to take precautions. You can check the authenticity of a bill by holding it up to the light and matching the watermark to the figure printed in the middle of the bill. Authorities also recommend using counterfeit detection pens on larger bills.