There was a thread on here a while ago about asking examiners to show their credentials before a checkride. I was amazed how many posted that they would never dare to ask and a few, presumably examiners who stated that they would take great offence.
If I have a pilot (anyone from student to examiner) coming to fly one of my aeroplanes I have a duty as the aircraft operator to ensure that he is properly quaified. If he can't or wont show me the proper paperwork he aint flying my aeroplane - simple!
Happy landings
3 Point