Many signs have been related signifying the reappearance of Imam Mahdi (A). Some of these signs are considered more fixed than others, and some of these narrations are considered more correct than others. In any case, the underlying theme is that the natural and political order of the world will be turned upside down, there will be chaos beforehand, and when he arises, it will be clear with no doubt. (Therefore, people who go on TV or online today and say they are the Mahdi are not really the Mahdi since they are not accompanied by signs.)
Some of the signs are:
* The rise of the Sufyani
* A solar eclipse the middle of the month of Ramadan and a lunar eclipse at the end of Ramadan
* Some of the land being swallowed up
* The sun rising from the west
* Black flags marching from Khurasan
* The killing of the "pure soul" and the killing of a Hashemite near the Ka'bah
* A star as bright as the moon
* A fire in the sky
* The Euphrates flooding into Kufa
* False prophets
* The revolt of the Yamani
* The people of Egypt will kill their ruler and destroy Syria, and three standards will fight over it.
* Locuts at the usual and not usual times destroying the food
* Two groups of foreigners will dispute and much blood will be shed
* A cry from the sky that all will hear
* A face in the sun.
There are a number of books on Imam Mahdi (A) including on this website and you can read more signs there.