There is no universal question.
I think the only effective question is one that matches their existing dissapointment or issue OR is something that they have never heard before and will want to research.
I discovered the 587 thing by myself 12 years ago and it never dawned on me to do confirm it because the Society obviously already researched it well for me. I also discoverd that our debunking of the Trinity was not the same Trinity that most Christians believe in about 10 years ago, again, didn't really bother me too much either. Although it might be a small thing, what shook me was that I felt that the society overstates the effectiveness of EPO after watching one of my friends die because they took EPO and iron instead of blood. If that didn't happen, I may have just kept shelving my doubts. To most other witnesses, I doubt EPO misrepresentation would be a big deal. To others 587 could be a big deal.