How can I know if a key is already used?

There is no possible way to check whether a key is valid without activating it. Steam will never create a system to check keys because, simply put, they don't want people trading keys. Games purchased from their store have the trading privileges.

Sorry, you're not the only one who wants it :(

invalid might mean you are locked out

If it says you already have the game, then the game you're trying is already in your library. Chances are the keys are unused. I doubt you get that message if the key is invalid.

You cant!

You can't check if keys are used or not

Hmm no idea Mizi, you'll be best not giving them away probably :| (you can trade, but you go first so they can check if the key works xD)

You can check if a key is used or not by contacting steam support. All they can tell you is weather the key is used or not. They can't tell you who used the key.

Just make a group giveaway with the disclaimer: I DON'T KNOW IF THE KEYS WORK!
And like iCybershot says, you can contact the Steam support to know whether the key is used, or unused.