In Jeremiah 1:2 it is made clear: to whom the word of the Lord came.
And that comes after Jeremiah 1:1: the words of Jeremiah.
It is also clearly shown in his calling: God wants to use him to pass on his message. And lastly, it is evident in the fact that in the period of the last kings of Judah, in 586 BC, the people were finally sent into exile, far away from their own country. His prophecy that this would happen came true.
2 to whom the word of the LORD came in the days of Josiah the son of Amon, king of Judah, in the thirteenth year of his reign.