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Sadly not.

no

You can if you already own the item in question, I think

well have your friend that already have the game try to activate it

short answer: there is no way to check a key in steam without redeeming it, the only thing you can to is to try and activate it in your account provided you don't have the game

Give the winner one key, if he says it doesn't work, check his profile and if the game REALLY didn't activate on his account you should give him the other key.

You can email steam support to ask them which key is unused.

Nope.

Edit: That's why use "Steam Gifts" rather than "Steam keys" :D