How can I tell if someone is truly Mahdi the savior?

I am doing a research and trying to figure out an answer to my question: Throughout centuries numerous people have claimed to be Mahdi the savior, some have been killed right away, some have died yet they have gathered some followers that are still following their claim. So how are we to distinguish a fake Mahdi from *The Mahdi* himself? What are the **key attributes** that once someone claims to be Mahdi we could assess and then say: "No! This person is a fake" Or "Subhanallah he is the Mahdi". Please give me answers that are logical, yet with proof from Quran and narrations, I would like to hear both Sunni and Shiite views. Just have in mind that I am considering that surely there would be a savior known as Mahdi.