I don't have a good question to ask (fortunately, my parents divorced when I was young and I went to the non-JW side of the family -- phew, close call) as I'm remarkably ignorant as to what is actually in the bible.
One thing is worth mentioning though, if you do choose to challenge a JWs beliefs in any way, do so as gently and kindly as possible, for a variety of reasons.
One is that you will have a greater impact on them if you do not "attack" them or their beliefs. JWs are used to being attacked and have automated mental defences against it. Best not to get their guard up, then they might accidentally listen. Mind control is largely about social protocol; The more they identify with you socially, the more impact you will have.
Another is for the sake of human decency. That poor sap actually believes that crap, otherwise there's no way s/he would ever tolerate the crappy predicament they're in. They're a victim, and victims need help and compassion.
Best thing to do is just ask them questions they can't answer. Be as gentle as possible and call them brother or sister, join them in their quest for "truth". Don't necessarily agree with them, just suggest that their version doesn't quite satisfy and maybe they should study a bit more. Make clear the distinction between them as individuals and the WTS. Let them recite WT BS, but know when they hesitate to answer or stumble over their words, you have successfully planted the seeds of doubt.