These teachers will say that they are anointed by God and are sharing the “true” gospel. Jeremiah faced this. Look at Jeremiah 28. The false prophet Hananiah, like Jeremiah, started his false prophecy with, “Thus says the Lord.” So both Hananiah and Jeremiah stated they were sharing the Word of the Lord but their messages were complete opposites. If you are one of the people of Judah how do you determine which one is lying and which one is telling the truth? Cause God is going to hold you responsible for getting it right.
Paul faced this. He was traveling around the Mediterranean with a new and offensive message. We get some instruction from the Berean Jews. Acts 17:11 says that they did not accept what Paul said on face value, no, they examined the Scripture to see if what he was saying aligned with the text. That was not a simple task. They did not have iPhones with Bible apps. To look at the Word they had to go to the local synagogue and go through the scrolls to check out what Paul was saying. Their effort was called by the Holy Spirit, noble minded.
Jeremiah 23:22 gives us another clue as to how to discern error, standing in God’s counsel. Coming before Him with our concern, our confusion, with His Word and asking Him to clarify for us. We have a significant advantage over both the people of Judah and the Berean Jews, if we have accepted Christ we have the Holy Spirit resident in us. John 16:13 tells us that one of the roles of the Spirit is to lead us into truth.
So it looks like to discern whether something a teacher is telling us is true we need to go to the Word, in the counsel of God, and be led by the Holy Spirit.