How to verify CDKEY

if they're steam keys, it's not possible without redeeming them.

I don think there is anyway to check if your (steam)CDKEY is good. As long as you bought it from a trustable store like GMG or steam it should be good

You can't do that on Steam. :(

I think you can open a support ticket, but not sure, and it might take you forever to get an answer... Good luck.

another discussion

tl;dr it's not possible

WOW guys youre fast, thanks for the nonimaginable quick respond :)

is not possible.

possible it is not

If you trying to use key for game you already have, steam says about it, so you can be sure key is real. But it seems you can't check is this key used or not.