I would suggest to prospective FI's, ask if your training is one to one, as my impression is they have quite a few instructors who are part timers and have 'day jobs'.
This is exactly what happened to me, and my examiner rating was cancelled with very short notice due to the instructor being called in for his day job.
Again if a FI instructor/examiner, claims to be ex military, don't fall the assumption that the were a pilot in the military, they may have been a navigator or rock ape! Of course I'm not thinking of anyone in particular!!!!
I've fallen for this one far too many times with people who sat in the back of a Nimrod.