*Edit* This may or may not work outside the US. I'm am not certain on how to detect counterfeits elsewhere but the concept is simple and could be molded to your area most likely.
Step 1: Material Needed
Step 2: What to Do
At this point the customer will ask why you just checked it or something along those lines. State why it's a counterfeit and say you can;t accept it. Trust me on this when I say this is the hardest step, You have to keep a straight face. The customer will be able to see it in your face or the way you speak and know right away that it's a prank.