True, @Vanderhoven, but the disciples asked two questions. "Tell us, when will these things be, and what will be the sign of your presence and of the conclusion of the system of things?" (Matthew 24:3) Jesus answered them both.
(1) When will these things be (regarding the first century temple being torn down)?
(2) What will be the sign of your presence in Kingdom power and the conclusion of the system of things?
Jesus told them when the Jewish system of things would end. But he also answered when he would come in Kingdom power and end the complete current system of things.
How do we know?
"...for then there will be great tribulation such as has not occurred since the world’s beginning until now, no, nor will occur again..." (Matthew 24:21)
The tribulation that occurred in Jerusalem in the first century was bad, but it's not the worst tribulation ever to occur on the earth. There have been plenty of bad "tribulations" since that time up until now. The great tribulation to come will be worse than what happened in Jerusalem, and it will be worse than anything that has happened up to this point.
"In fact, unless those days were cut short, no flesh would be saved; but on account of the chosen ones those days will be cut short."
Also, these things haven't happened yet...
"Immediately after the tribulation of those days, 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 the sign of the Son of man will appear in heaven, and all the tribes of the earth will beat themselves in grief, and they will see the Son of man coming on the clouds of heaven with power and great glory. And he will send out his angels with a great trumpet sound, and they will gather his chosen ones together from the four winds, from one extremity of the heavens to their other extremity."
Everybody will see Jesus. Everyone will know. We won't have to say, "hey, did you know this means Jesus is present?" Nope. Everyone will know without needing to be told. "They will see the Son of man coming on the clouds of heaven with power and great glory."