How can you recognize if something that is said about God is really true?

Luther:

Let me put it this way: heretics always say what people want to hear. They do not go against opinion, or above it, but what they say joins with it. But the true Word of God always goes against our own feelings and what we like. It does not let our beliefs stand, but brings them down and breaks them down. So, when someone claims that God’s Word fits with what the people want, then you can rest assured that it is not true. Then you make God into merely what men want.