As the title says, I'm wondering if there's any way to check if a Steam key has already been used. In my experience Steam will display the same message if you try activating a key for a game you already own, no matter if the key is still valid or has already been used. And in my experience you also can't test the validity of a key for a game you don't own yet without instantly redeeming it to your account. (another thing our cosy little GOG does so much better than Gaben's multi billion dollar abomination)

The thing is that I still have a buttload of Steam keys for all sorts of games and I can't tell (and sure as hell don't remember) which ones I've already redeemed / given away. Most of them are still valid and I'd gladly give some of them to GOGers but of course I won't host giveaways if I can't tell if a particular key is still valid.