Perry on Politics interviewed Andrew Sullivan and asked this question:
TP: Having been a proponent of gay marriage, what is the best argument against it that you have heard? And what is your response to that argument?
One could ask a candidate about an important decision she made and then ask what was the best argument against her point of view.
This would reveal:
1. Her depth of understanding of the issues.
2. Her comfort level in looking at things from someone else's perspective.