I suspectthat in order to do this you will have to check more than one parameter. I do not know about the rest of the world , but here in Canada the mint changes the alloy of the coins.I believe that the one thing they do adhere to is that the weight of each denomination is controlled.
If this is true for the coins you want to detect then weight would be usable as one of the test parameters with thickness or diameter as secondary parameters you should be able to eliminate 99 + % of false coins. hth Terry