It will to a point, If you own the game and as long as it is valid it will say you already have the game, It may however already have been used, and will not tell you otherwise until entered into an account without the game.
And if it comes up not valid, regardless of if you have the game or not its not valid, simple.

I would not recommend giving away keys you win, for the above reasons, at least not on giveaway sites. I just throw them at my brother and friends, the lucky SOB's have got so many games that way ;)

Thanks for the info, I entered it in the client and I got a failure that I already own Grimm, so it should be working :)