You don't. Even better, this would be great as a scam, if you simply count on it being handed a couple of times from person to person before someone decides to check the code online. Who are you going to complain to if it turns out not to work at redemption time?

Would you accept one of these without verifying that it has indeed not been redeemed. How do you know that your coin is the only coin with that particular code in it without redeeming it?