Okay, I will bite

We will know because he will come in the clouds as KING OF KINGS and defeat those that are in battle. He will come with all HIS saints in Heaven.
Matthew: 4And Jesus answered and said unto them, Take heed that no man deceive you.
5For many shall come in my name, saying, I am Christ; and shall deceive many.

then on to Matthew: 30And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory.

31And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other.