First and foremost if they understand the basics, one God in 3 persons, salvation through grace alone, and the Bible is the word of God. We can argue all day about the other nuances. And to start that would be to ask questions. Understand what they are reading and how they are defining what they read. Then going back to see what scripture says and maybe their is multiple ways to understand something. From there again keeping it civil never rising to anger because we can argue how many angels fit on the head of a pin but without true salvation it means nothing.