query about CD-KEY [NO GA cause hate]

The person entering the key does not get any feedback except for the mail, so a general activation check is not possible... You can only check if it was activated to YOUR account

Sadly no, you can't check if a key is used or not. You can only check what account a Cd-key was used for when you try to activate an already used key, and to my knowledge, all that really happens is that the account owner gets an email like that.