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?
Not exactly, because of the way public addresses and private keys work.
Anyone can check that the address 1xGibi... holds the declared amount of bitcoins. Only someone with the knowledge of private key can transfer them to other address. So you can verify the address and physical integrity of coins before accepting them.
You may think of this as of passing around a signed check.
