Surprisingly, the question I asked which revealed to me that the OW is not 'just a friend', despite him vehemently protesting that she was, was: does she know my name?
His response was '... (pause), probably... (pause), I don't know'.
I found this kinda strange considering I'd just been told that they had been er 'friends' for years.
Guess I asked a question he wasn't expecting.
Are there any other key questions others have found to be illuminating?