Checking if a steam key works?

I won a copy of Shadowgrounds here, and redeemed it. Then, the next day, I won a copy on summerseal. I tried to just give it away on Traders Guild forums (like I've seen other people do) but I'm not sure if any one took me seriously. Is there a way to test if it was used or not without redeeming it? When I try to redeem it on my account it says I already have it instead of "Duplicate product key" so does that mean it's still good?