Sometimes you come across an iTunes promo or gift card code, but don't know if it's still valid or not. Here's a sneaky way to check.
Normally, the only way to find out is to try and redeem it, but that ruins any chance of letting somebody else use it. Instead, try logging out of iTunes, and if you have a "save password" option, log back in to uncheck the option before logging back out again.
Quit iTunes once to be safe, then reopen it and try to redeem the code without logging in first. If the code has already been used, you'll get a message telling you so, but if it's still redeemable, iTunes will prompt you for your password—which is where you press cancel to keep it that way.