The prophet which prophesieth of peace, when the word of the prophet shall come to pass, then shall the prophet be known, that the LORD hath truly sent him. When the word of the prophet shall come to pass - Here is the criterion. He is a true prophet who specifies things that he says shall happen, and also fixes the time of the event; and the things do happen, and in that time.
You say that Nebuchadnezzar shall not overthrow this city; and that in two years from this time, not only the sacred vessels already taken away shall be restored, but also that Jeconiah and all the Jewish captives shall be restored, and the Babylonish yoke broken, see Jeremiah 28:2, Jeremiah 28:3, Jeremiah 28:4. Now I say that Nebuchadnezzar will come this year, and destroy this city, and lead away the rest of the people into captivity, and the rest of the sacred vessels; and that there will be no restoration of any kind till seventy years from this time.