Can I check if a key is valid or fake/used without adding to my Steam Account?

I thought it would say "product owned" even if the key is dupe

I wonder if a key I have for Spec Ops: The Line would be able to be given away then.. I put it into Steam during a brain fart thinking it would ask me if I would like to put it in my inventory. Instead, it said it activated it... but I already owned the game (bought it through Steam, key was through Amazon). I was kinda mad.

no.

there's always one ...

Nice try.

They will never check any lol. According to Steam, keys are not meant to be traded or exchanged, only activated. Therefore they would tell you to try activating the key yourself and see if it works.

I'm not so sure about that...I have a few keys lying around that I've gotten from facebook giveaways or some such. Nothing major - just Metro, Crayon Deluxe and that Fairie Solitaire. Maybe 1-2 more. Things that were given away by the thousands but I kinda let rot in my inbox as it was all during the holidays and I didn't set that stuff as a priority. I'd like to put them up for trade or whatever but since I'm only 95% sure they'e still valid, that 5% is holding me back since I don't want to be accused of being a scammer. I think I can at least put Crayon Deluxe up here but again, don't wanna take that chance now.

It'd be nice if I can verify via Steam that the keys are still good....otherwise I'd probably just do a ninja key drop at some point with that caveat that the codes may not work and then be accused of farming for rep with fake keys (on other sites, not specifically here). Ah well.