Perhaps after all of this discussion we may find that a person's interaction with other crew members and the "gut level" feelings they induce may be the most sensitive test of their mental condition. One interaction should not be conclusive but if several associates provide the same opinion then perhaps deeper investigation is warranted. I know that our airline had "Professional Standards" representatives in the union branch who you could approach to discretely report any undesirable attitudes by other crew members. Reports could then be dealt with on a peer-to-peer level but ultimately they could also be escalated to the company. This system was introduced to deal with some of the disastrous CRM failures of the period.
I know that personally I found out far more about other crew member's personal lives during long flights and beers in the bar on layovers than I wanted to know. My joke was that the worst words you could hear on the flight deck on a long flight was the Captain saying "Did I tell you what my wife's lawyer did to me last week?".
On one occasion I was able to help somebody out as a result of the bar debrief. The co-pilot I had been flying with for a couple of weeks suddenly became distracted and less chatty. Over a beer it transpired that his sister was involved in a bad case of domestic violence and they were arranging an escape from the husband, the co-pilot was concerned that the husband would try to take revenge while he was away. Being relatively new with the company he was concerned that his case would not justify company compassion. Easy fix, at end of trip I walked in with him to see a flight manager buddy and he walked out with time-off until the immediate problem was taken care of. I hope present airlines would be so flexible and sensible.
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