New Revised Standard Version
When this comes—and come it will!—then they shall know that a prophet has been among them.

Contemporary English Version
Soon they will be punished, just as you warned, and they will know that a prophet has been among them.

New American Bible
But when it comes—and it is surely coming!—they shall know that there was a prophet among them.

Douay-Rheims Bible
And when that which was foretold shall come to pass, for behold it is coming, then shall they know that a prophet hath been among them.

And when that which was foretold shall come to pass, for behold it is coming, then shall they know that a prophet hath been among them.

when

1 Samuel 3:19,20 And Samuel grew, and the Lord was with him, and not one of his words fell to the ground. . . .

Jeremiah 28:9 The prophet that prophesied peace: when his word shall come to pass, the prophet shall be known, whom the hath sent in truth.

shall

Ezekiel 2:5 If so be they at least will hear, and if so be they will forbear, for they are a provoking house: and they shall know that there hath been a prophet in the midst of them.

2 Kings 5:8 And when Eliseus, the man of God, had heard this, to wit, that the king of Israel had rent his garments, he sent to him, saying: Why hast thou rent thy garments? let him come to me, and let him know that there is a prophet in Israel.

Luke 10:11 Even the very dust of your city that cleaveth to us, we wipe off against you. Yet know this, that the kingdom of God is at hand.