Your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions:

I have heard many repeat this phrase from scripture:
And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions: (Joel 2:28)

But this event has not yet arrived. How can you tell? The phrase begins with "it shall come to pass afterward." After what?

Joel 1 and the part of Joel 2 that occurs before this describes a foreign invasion. Not just any foreign invasion, either. Here are some segments. Starting in Joel 1:4:

 4 That which the palmerworm hath left hath the locust eaten; and that which the locust hath left hath the cankerworm eaten; and that which the cankerworm hath left hath the caterpiller eaten.
 5 Awake, ye drunkards, and weep; and howl, all ye drinkers of wine, because of the new wine; for it is cut off from your mouth.
 6 For a nation is come up upon my land, strong, and without number, whose teeth are the teeth of a lion, and he hath the cheek teeth of a great lion.
 7 He hath laid my vine waste, and barked my fig tree: he hath made it clean bare, and cast it away; the branches thereof are made white. (Joel 1)

Envision the imagery here. God describes one wave of destruction after another. With each successive wave, whatever is left is consumed. In the end, only the gleanings of the gleanings of the gleanings is left! Verse 7 is easier to visualize. Imagine a fruitful tree. First, someone comes along and tears off the fruit and carries them away. Still, the tree is covered with lush foliage. Next, someone comes along and tears off all the leaves. The tree is now undeniably naked compared to its original state. But the hand of the Lord is outstretched still. He sends someone else to peel the bark off the tree ("the branches thereof are made white). It is hard to create more vivid imagery of complete and utter destruction than this!

 8 ¶Lament like a virgin girded with sackcloth for the husband of her youth.
 9 The meat offering and the drink offering is cut off from the house of the Lord; the priests, the Lord’s ministers, mourn.
 10 The field is wasted, the land mourneth; for the corn is wasted: the new wine is dried up, the oil languisheth.
 11 Be ye ashamed, O ye husbandmen; howl, O ye vinedressers, for the wheat and for the barley; because the harvest of the field is perished.
 12 The vine is dried up, and the fig tree languisheth; the pomegranate tree, the palm tree also, and the apple tree, even all the trees of the field, are withered: because joy is withered away from the sons of men.
 13 Gird yourselves, and lament, ye priests: howl, ye ministers of the altar: come, lie all night in sackcloth, ye ministers of my God: for the meat offering and the drink offering is withholden from the house of your God.
 14 ¶Sanctify ye a fast, call a solemn assembly, gather the elders and all the inhabitants of the land into the house of the Lord your God, and cry unto the Lord,
 15 Alas for the day! for the day of the Lord is at hand, and as a destruction from the Almighty shall it come.
 16 Is not the meat cut off before our eyes, yea, joy and gladness from the house of our God?
 17 The seed is rotten under their clods, the garners are laid desolate, the barns are broken down; for the corn is withered.
 18 How do the beasts groan! the herds of cattle are perplexed, because they have no pasture; yea, the flocks of sheep are made desolate.
 19 O Lord, to thee will I cry: for the fire hath devoured the pastures of the wilderness, and the flame hath burned all the trees of the field.
 20 The beasts of the field cry also unto thee: for the rivers of waters are dried up, and the fire hath devoured the pastures of the wilderness. (Joel 1)

The first phase of destruction comes with a terrible famine. Joel describes that there is no water and no food. The land is so dry that the wilderness has caught fire.

It is this with this backdrop that the foreign invasion occurs.

1 Blow ye the trumpet in Zion, and sound an alarm in my holy mountain: let all the inhabitants of the land tremble: for the day of the Lord cometh, for it is nigh at hand;
 2 A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread upon the mountains: a great people and a strong; there hath not been ever the like, neither shall be any more after it, even to the years of many generations.
 3 A fire devoureth before them; and behind them a flame burneth: the land is as the garden of Eden before them, and behind them a desolate wilderness; yea, and nothing shall escape them.
 4 The appearance of them is as the appearance of horses; and as horsemen, so shall they run.
 5 Like the noise of chariots on the tops of mountains shall they leap, like the noise of a flame of fire that devoureth the stubble, as a strong people set in battle array.
 6 Before their face the people shall be much pained: all faces shall gather blackness.
 7 They shall run like mighty men; they shall climb the wall like men of war; and they shall march every one on his ways, and they shall not break their ranks:
 8 Neither shall one thrust another; they shall walk every one in his path: and when they fall upon the sword, they shall not be wounded.
 9 They shall run to and fro in the city; they shall run upon the wall, they shall climb up upon the houses; they shall enter in at the windows like a thief.
 10 The earth shall quake before them; the heavens shall tremble: the sun and the moon shall be dark, and the stars shall withdraw their shining:
 11 And the Lord shall utter his voice before his army: for his camp is very great: for he is strong that executeth his word: for the day of the Lord is great and very terrible; and who can abide it? (Joel 2)

The destruction by the invading army is complete. It is beyond the capacity of most in the United States to imagine just how complete the destruction will be. Joel provides vivid language in an attempt to help us. First comes something to make the earth shake (verse 10). Then comes a trembling of the heavens (verse 10). Whatever this is causes the sun and moon to be dark, and the stars to stop shining. The stars will not actually stop shining at this point, nor will the sun and moon stop shining. Instead, the darkness in the atmosphere will block out the light of all heavenly bodies. And a great earthquake will occur in conjunction with the solar event. This is the opening of the sixth seal in the book of Revelation.

Perhaps you cannot imagine the invading army marching in unison across the country. Here is a thought experiment. The United States is 1,650 miles at the greatest North-South distance. Do you know that China and Russia have enough tanks to sweep the United States east to west with 15 tanks per mile? Obviously, land features would prevent them from literally driving from one end to the other, and this would not be the best way to invade a country, but the point is: they have the hardware to do it. Think about that.