This will work on $10 bills and $20 bills. I don't know if this applies to higher denominations like $50 bills and $100 bills. Please reply if you do know. This will not work on $1 bills, $2 bills, and $5 bills. Hold the dollar bill up to the light and if you see the President's face on the right side of the bill, it's authentic. If you don't see the President's face it's counterfeit.