Thank you for your question. We have traditions explaining events before the appearance of Imam al-Mahdi (af) and surrounding the way in which he will reappear. These hadith which are prophecies will be one of the best indicators for knowing the Imam (af). Some people will be blessed with knowing him before his reappearance and those are his selected companions who will travel to Mecca and be present when he reveales his identity.
Aside from these there are a number of traits and qualities that an Imam must have and Imam al-Mahdi (af) will display those traits. He will also most likely display proofs of his own as did the previous divine guides.
May you always be successful.