John Gill
and it may be observed, that the other evangelists make no mention of the sign, only speak of the son of man, Mar_13:26 and he shall appear, not in person, but in the power of his wrath and vengeance, on the Jewish nation which will be a full sign and proof of his being come: for the sense is, that when the above calamities shall be upon the civil state of that people, and there will be such changes in their ecclesiastical state it will be as clear a point, that Christ is come in the flesh, and that he is also come in his vengeance on that nation, for their rejection and crucifixion him, as if they had seen him appear in person in the heavens. They had been always seeking a sign, and were continually asking one of him; and now they will have a sign with a witness; as they had accordingly,
Adam Clarke
Then shall appear the sign of the Son of man - The plain meaning of this is, that the destruction of Jerusalem will be such a remarkable instance of Divine vengeance, such a signal manifestation of Christ’s power and glory, that all the Jewish tribes shall mourn, and many will, in consequence of this manifestation of God, be led to acknowledge Christ and his religion. By της γης, of the land, in the text, is evidently meant here, as in several other places, the land of Judea and its tribes, either its then inhabitants, or the Jewish people wherever found.
Albert Barnes
The sign of the Son of man - The "evidence" that he is coming to destroy the city of Jerusalem.
Caiaphus was told he would see this event:
Mat 26:64 Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man sitting on the right hand of power, and coming in the clouds of heaven.
Again John Gill commenting on verse 64:
and coming in, the clouds of heaven. So Christ's coming to take vengeance on the Jewish nation, as it is often called the coming of the son of man, is described in this manner, Mat_24:27.
The context of the Discourse:
Mat 24:34 Verily I say unto you, This generation shall not pass, till all these things be fulfilled.