Jesus referred to many signs of his return in the Olivet prophecy, recorded in Matthew 24, Mark 13 and Luke 21.
The unmistakable signs are given in Matthew 24:29-30,
“The sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken. Then will appear in heaven the sign of the Son of Man coming on the clouds of heaven.”
Until that time comes, the greatest sign is the return of the Jews from all countries back to Israel. This was predicted by Jeremiah about 2,500 years ago, and is closely associated with the kingship of Jesus Christ. For example,
“Behold, the days are coming, declares the LORD, when I will raise up for David a righteous Branch, and he shall reign as king and deal wisely, and shall execute justice and righteousness in the land. In his days Judah will be saved, and Israel will dwell securely. And this is the name by which he will be called: ‘The LORD is our righteousness.’
“Therefore, behold, the days are coming, declares the LORD, when they shall no longer say, ‘As the LORD lives who brought up the people of Israel out of the land of Egypt,’ but ‘As the LORD lives who brought up and led the offspring of the house of Israel out of the north country and out of all the countries where he had driven them.’ Then they shall dwell in their own land.”
Similarly, Ezekiel 37 links the return of the Jews to Israel (Eze 37:1-14) with the return of Jesus and the establishment of the kingdom of God on earth (Eze 37:24-28).