There are many non definite signs of the reappearance of Imam Al-Mahdi (AS) like:
1. A war which takes lives of two thirds of the people.
2, Demolishing if the wall of Masjid Al-Kufa.
3. Sever drought before the reappearance.
4. Earthquakes and plague in many places.
5. Black flags coming from Khurasan.
6. Injustice and wrong filling the earth.
7. Reddish colour appearing in the sky till it fills the whole sky.
8. Destruction of Buratha Masjid in Baghdad.
9. Fire between the earth and the sky.
10. Appearing the water from Najaf sea.
And many other non definite signs mentioned in narrations.
Wassalam.