How do I know if humble bundle key is unused?

Actually the blue text might indicate that it is a gift link not a key.

In case it's not a gift link you can make the giveaway for this group or make a public one but put a warning in the description, although not all winners of public giveaways will agree to delete it if the key is invalid.

Humble doesn't tell you if the key has been used or not. If it shows a key in green text with a redeem button(if you have your steam account linked) that means you revealed the key. If it shows a blue text with a link you made a gift link. the only Real way to know the key is not used is if it still says Reveal key or gift to friend.