One characteristic that will establish Hazrat Mahdi's (as) identity is his descent from Hazrat Hassan (ra)

“HAZRAT MAHDI (AS) IS A CHILD OF FATIMA AND DESCENDED FROM HASSAN." (Sunan Abu Dawud, Hazrat Mahdi (as), 1) 

Abd al-Ghafir al-Farisi in his book Majma al-Gharaib and Ibn al-Jawzi in fi Garib al-Hadith and Ibn al-Athir in al-Nihayah have stated that Hazrat Ali (ra) said this about the hadith:

HAZRAT MAHDI (AS) IS DESCENDED FROM HAZRAT HASSAN. (Al-Muttaqi al-Hindi, Al-Burhan fi Alamat al-Mahdi Akhir az-Zaman, p. 22) 

Tammam in his Fawa'id and Ibn al-Athakir relate from Abd-Allah ibn Amr that he said: A CHILD OF HASSAN [HAZRAT MAHDI (AS)]will appear from the east, and even if mountains stand against him, he will crush them and use those mountains as roads. (Al-Muttaqi al-Hindi, Al-Burhan fi Alamat al-Mahdi Akhir az-Zaman, p. 22) 

According to another hadith that has come down from Hazrat Ali, he looked at his son Hazrat Hassanone day and said:  “As named by the Prophet (saas), this son of mine is a sayyid. Someone [Hazrat Mahdi (as)] known by the name of the Prophet WILL COME FROM THE LINE OF HASSAN, and will resemble him [the Prophet (saas)] in moral values, but not in his nature (physical and body features).” (Al-Taj V, 363)