When obito confronted the five kages in the five kage summit arc why did they believe him that he is madara and why didnt say ask him to take out the mask or proof???they should at least say prove it
Basically I think they figured that if he was willing to announce in front of the Kage that he was Madara, only the real Madara would have the balls to do that.
Well, "The Power in a Name" basically states how much fear Madara made just by his name alone because of his actions. The saying "action speaks louder than words" is pretty the case in a sense.
as tsunade stated his name has power they were all duped and since madara gave obito info that only very few knew of course they would then believe it and then he tells them about the juubi which further makes them believe