Here are some questions to ask visiting JWs to help them realize their faith is not in the Bible, but in men.
QUESTIONS ABOUT HOW YOU VIEW THE BIBLE.
1. Do you view the Bible as the inspired word of God? (II Tim.3:16)
2. Do you believe the Bible is written for everyone? (to read and study)
3. Do you believe that someone who only has the Bible can read it and be saved?
4. Do you believe your religion is clearly found in the Bible?
5. Could someone studying the Bible alone come up with exactly what you believe?
6. What role does the Bible play in determining your religious beliefs?
7. Do you believe that your faith is primarily in the Bible or is it in your religious leaders and their interpretations of the Bible?
8. Could there be a discrepancy between what your religion teaches and what the Bible teaches? Has there ever been?
9. If you discovered a discrepancy between what the Bible says and what your religion teaches, which one would you believe?
10. Would doctrinal modifications made by your religious leaders always act as a corrective to change what you believe the Bible teaches?
11. Could the Bible ever act as a corrective to change your beliefs contrary to what your religion teaches?
12. If a person puts his complete trust in a Bible scholar, with an “if he says it, I believe it” attitude, would his primary faith be in the Bible or the scholar?
13. Do you believe that your religious leaders alone have the right and ability to to interpret the Bible for true believers?
14. Where do your leaders get new information as to what the Bible teaches? (Bible itself? Angelic guidance? heavenly saints? Holy Spirit's leading?)
15. How do your leaders decide on which doctrinal changes to adopt? Do they sometimes vote changes in?