Confirming game keys

You cannot
Also check this

Unfortunately, you cannot check if a key has already been used

oh no....and what happens if I create the giveaway and it turns out it has been redeemed?

why dont you just post them ... I know it sounds dumb but it lowers the risk to loose rep

if you warn people first and then post it on forum, i don't think that'll hurt your rep, just don't make a regular giveaway.

post it here like this :P - 4VBDH-7WIVD-ET07D

Ask Steam support

You can give them to me and i´ll chek it for you :P

in Origin you can, in Steam you can't.

You can check if you own the game already, cant you? It just tells you that you already have the game, not sure if that verifies its redeemable though..