how do I find out what a key is for without activating it?

you can't.

There's no way to find it out yourself.

Don't worry about it. If something like that even existed, by the time you ran the key to find out what it was, someone else would have taken the game already anyway.

that is unfortunate.

have every game on steam
try a key
????
profit

Valve employees and official retailers have that tool for checking keys

That's one thing feature I miss from the PlayStation Network and Xbox LIVE. I've a few Steam-redeemable keys from 2010 and no idea what they are anymore.

The only ones I've found out to be reliably easy to figure out is Borderlands 2, as all the keys end in "BL2" such as this one: 9GUAA-3FARX-9NCRK-9EXEX-3WBL2 [used already]