Welcome to the group.
This was a very general question, so I thought a simple answer referring to the scriptures that use the phrase "Son of Man" and which combine "sign" and "son of Man might be helpful.
In Daniel 7:11 we read: "I saw in the night visions, and behold, with the clouds of heaven there came one like a son of man, and he came to the Ancient of Days and was presented before him. (ESV)
And we see Jesus use this imagery when He says in Matthew 24:30 "Then will appear in heaven the sign of the Son of Man, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory. (ESV)
Again we see in Matthew 26:64 Jesus said to him, "You have said so. But I tell you, from now on you will see the Son of Man seated at the right hand of Power and coming on the clouds of heaven." (ESV) Then, in Matt. 26:65 the High priest tears his robes (which is actually a violation of the Levitical law) he declares "He has uttered blasphemy. What further witnesses do we need? You have now heard his blasphemy." (ESV) (It appears that the priest at least understood that Jesus was appropriating the sign of the Messiah's return in Daniel.)
This also lines-up with the prophecy of the angels who accompany the disciples at the ascension of Christ in Acts 1:9-11. After Jesus "was lifted up, and a cloud took him out of their sight. And while they were gazing into heaven as he went, behold, two men stood by them in white robes, and said, 'Men of Galilee, why do you stand looking into heaven? This Jesus, who was taken up from you into heaven, will come in the same way as you saw him go into heaven.' "(ESV)
For these reasons, I believe that the sign of the Son of Man is the appearance of Jesus, the Messiah, returning to the Earth, for all to see.