When  Imam Al-Mahdi (peace be upon him) appears, he will be accompanied by a set of cosmic signs known to the whole world, including the rising of the sun from the west, and the huge sound from the sky. If will be appeared to the public and he will say: I am the Mahdi from the family of Muhammad (peace be upon them), because with the emergence of these two great signs, it is impossible for the impersonators to look like his personality.  Mam Al-Mahdi (AS) is supported by the divine support from Allah, and he will have the ability to practically eliminate his  opponents in the East and West.

Wassalam.