How can I check if the key isnt used?

Nope, Unfortunately not.

Exactly, there's no way to find out. Check Your Steam library for a certain game, that's all You can do.

Not without trying to activate it though doing so would then make the key invalid.

If you're not sure then I wouldn't recommend that you try to give it away here.

I'll check it for you ;D

Well, I hope that it's not taken! I'll give it away!

In my experience, you can't. :(

If you try the key but you already own the game, you get the "product already owned" message - same message whether the key is used or unused. If you try to test an unused key on your account and you already own the game, the key will remain unused and someone else can still use it.

If you try the key but you don't already own the game, a used key will give you "duplicate product code" - but unfortunately a good key will add the game to your account. Oops. :)

There is a way.. But it's terrible. I had this problem before so, I contacted Steam's support and got a answer within some days. Try your luck.

I would suggest to avoid making the giveaway if you're not sure...or doing it if you are ok to find another copy of the game to give away.

Just do as EndDead said, you could contact steam support before making giveaway if you are not sure. Steam at this point sucks. You can check origin cd key on origin without directly activating it, and even the crappy uplay can do it but for a platform which has existed for 10 years, it is not able to do so? This is a huge joke.