Lol. The ethics of MBTI state that it is the test takers' responsibility to decide which type they are. If the test taker wants to be accurate, then it would be wise for the test taker to fully understand MBTI and the concepts surrounding it so they can draw a conclusion on their own terms rather than someone else's, since their motivations are self-evident to them and no one else's.
While the conclusion drawn from a bystander has the potential to be correct, from a theoretical 1/16, their process in doing so will be flawed. It is tantamount to a guess, a crap shoot, or a gamble.