I was OIC a police Fraud Squad for about 10 years. One of the techniques we used when interviewing people was to 'play dumb' and ask several times for simplified explanations. It usually became readily apparent when people couldn't explain that i) they didn't understand it themselves; ii) they were inconsistent because they were telling lies or making it up as they went along. (Oh, the fun we had!)
It seems to me that this technique could usefully be applied to JWs.