Question about Steam Keys

Yes

yesh

The only way to know for certain is to try to redeem it on an account that doesn't own the game.

If you already own the game, it will say "product already owned" even if the key was already used.

It used to be that it said key was used before it checked if you already own the game. So you could check it on accounts that already own it, But valve didn't like it i guess. Plus some keys activated even if you owned game already. Had to make support ticket to free key like that once. Had same game added twice to my account heh.

Try it, thats it.