How do I find out the the game from a Steam Key without activating?

You dont

Give it on forum for ninjas

I have 2 games =) i can know. add in steam

basically what verygoodskill sort of said...if you know and trust someone who has both games when they try to activate the key it will just try to install the game it belongs to leaving the key active and they can tell you which game it is for

I think this is something Steam should really safe guard against since trading is now a thing. I have't personally had a problem with this but it would be nice to be able to enter a code and see if it's used or not without using it.