What i dont get is, even if you knew when or where your theme was activated, how would you know if it is stolen or bought?
Exactly. Even if you found a site using your theme there is no way to tell weather or not the person paid for it. I guess you could email every single person / site you find with your theme on it asking for a license number but chances are most people would just tel you to f-off anyways. You could protect your live previews from being ripped but when it comes down to the theme there really is no way to know.
I am not sure about the TF forums but I know a lot of WordPress plugin devs on CodeCanyon including myself have been pushing Envato for a built in way to handle theme / plugin activation and updates through the Envato Network.