How I know when the messiah comes?
The Messiah is destined to reestablish the royal dynasty of David, to rebuild the Temple, and gather in the scattered people. He will also re-introduce those halachos that we cannot fulfill today due to lack of knowledge (such as the yovel year), so that the fulfillment of the Torah will be rectified to a degree of perfection. Somebody who achieves all of this is certainly the Messiah. Somebody who goes some way toward achieving this, depending on how far he gets, can be assumed to be the Messiah.
Sources: Rambam, Laws of Kings, chap. 11.