Many, many, many years ago, I used to get the feeling that I wasn't talking to the bloke actually flying the aeroplane. It was generally East European airlines and it was the abnormally long delay between giving an instruction and getting a response that gave the suspicion. It could be quite unnerving when there were a lot of active strips on your desk. An explanation I was given was that often the only english speaker on board was a steward. How true that was, I don't know but it certainly felt possible.
GBZ