How do I know if a iTunes card has already been used
I found a card in my desk, but I'm not absolutely sure it's been used. How do I check it out?
iPad, I want to buy some books and albumn
The only way to know for sure is to try redeeming it onto an account, there isn't a site that you can use to check it it has or hasn't. If the card still has the strip on it that you need to remove to reveal its code then it probably hasn't been redeemed (unless somebody has managed to put the strip back on).
Redeeming a card : http://support.apple.com/kb/HT1574