Someone may have already asked this but...

I don't believe so.

:/ Nope.

I wish this existed but there is no way to check a key. If you own the game it will just try and install it, if you don't own the game it will use the key.

no :/

So... what do you think? Should I try gifting them even though I'm not pretty sure that they'll work? I don't know Y_Y. There's a high chance that they're working, but I don't want a lovely ban on the website for this annoying thing that I can't control xD

If you already have the game, you should be a be to check without using them up (it will display the name of the key)