I think you've missed my point. I was saying - or intending to - that being an FI doesn't mean I can do a few circuits with someone and tell if they'll be OK on, for instance, a long cross country. I haven't been trained to do that. I think I have as much chance of being able to do so as an experienced PPL; no more, no less
No instructor or examminer operates with the aid of a crystal ball.
If all you do with the pilot being checked is a few circuits then all you have to make a judgement on is their ability to safely take-off, perform the basic flight manoeuvres and land the aircraft.
We are talking about licensed pilots here, not students. All I expect of a pilot undergoing a check is to show me that they are not going to be a danger to themselves, their passengers, other operators or the club finances (damage the aircraft).
As an FI, one has experience of checking and correcting faults. One has also been licensed as an FI based in part on one's ability to make a decision regarding the competence of a pilot - that is what being an FI is all about.
When I fly with pilot's requiring a check, I can generally tell what type of a flight it is going to be based on how the pre-flight and pre-take-off checks are completed!
Regards,
DFC