You can't prove it with 100% certainty either (there's always the brother/sister/spouse/mother/dog/goat excuse) so the whole exercise is quite pointless. All we can do is try to audit Sportsbet's multi-account detection by demanding proof from them but since that has privacy implications and it wouldn't convince anyone who thinks that Sportsbet is lying (faking the proof) - I don't really see much use for that.