Well we need to understand some old Testament terms.
We tend to hear "coming" language and assume it means when he comes to resurrect the dead.
But the Old Testament has another type of coming.
A coming of God in judgement.
God came in judgement of many nations in the OT.
I think that's what Jesus is referencing here.
Not the final coming to raise the dead, but a coming in judgement.
I believe that coming in judgement happened in the destruction of Jerusalem in AD 70.
The temple was destroyed.
Those who ran the temple system were major enemies of Jesus.
They fought to have him crucified.
They lied about his resurrection.
So a clear sign that he wasn't overcome by them, would be when he came and destroyed their system through the Roman armies in AD70.
When he says they would see him coming in glory, what he means is they would perceive that he won against those who had ordered his death. With their system destroyed, they see that they had lost. Their lies about his non-resurrecting would be clearly perceived due to their destruction.
Their prophesied destruction would be verified, once they were indeed destroyed.
Jesus gave many indications that the very temple system he looked at would indeed be destroyed.
Luke 21
5 Some of his disciples were remarking about how the temple was adorned with beautiful stones and with gifts dedicated to God. But Jesus said,6 “As for what you see here, the time will come when not one stone will be left on another; every one of them will be thrown down.”
Matthew 23
29 “Woe to you, teachers of the law and Pharisees, you hypocrites! You build tombs for the prophets and decorate the graves of the righteous. 30 And you say, ‘If we had lived in the days of our ancestors, we would not have taken part with them in shedding the blood of the prophets.’ 31 So you testify against yourselves that you are the descendants of those who murdered the prophets. 32 Go ahead, then, and complete what your ancestors started!
33 “You snakes! You brood of vipers! How will you escape being condemned to hell?34 Therefore I am sending you prophets and sages and teachers. Some of them you will kill and crucify; others you will flog in your synagogues and pursue from town to town.35 And so upon you will come all the righteous blood that has been shed on earth, from the blood of righteous Abel to the blood of Zechariah son of Berekiah, whom you murdered between the temple and the altar. 36 Truly I tell you, all this will come on this generation.
Jesus clearly prophecied the destruction of that very temple, and a judgment upon that generation. All which would be a sign of his glory, and that he indeed was on the throne, and wasn't defeated by those who crucified him, and denied his resurrection.
Daniel 7
13 “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.
14 And to him was given dominion
and glory and a kingdom,
that all peoples, nations, and languages
should serve him;
his dominion is an everlasting dominion,
which shall not pass away,
and his kingdom one
that shall not be destroyed.
....
1 Peter 3
21 Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a good conscience, through the resurrection of Jesus Christ,22 who has gone into heaven and is at the right hand of God, with angels, authorities, and powers having been subjected to him.
Scripture clearly shows Jesus is at the right hand of God now, with all subject to him.
The AD 70 judgement fulfills Matthew 16:28.
They see him glorified over those who ordered his death, and denied his crucifixion.
He foretold it, and it was verified.
They could look back at his prophecy, see the destroyed temple, and know Jesus' words did indeed come true, as those who thought they won had clearly lost.
This all happened while some in that generation remained alive.