I think we have all seen the happenings described above. I have little sympathy for those crew members who party hard and end up being a flight safety hazard, but unless someone turns up for a flight obviously under the influence of something, there is little one can do unless the person in question declares themselves unfit. There is little of that done in my experience as the logical conclusion to persistant declarations of "unfit due to severe party" would doubtless be a precursor to a one sided conversation with the chief pilot, followed by directions to the door. In my experience, it didn't happen very often.
However, I have also seen genuine fatigue due to a series of night flights and disturbed sleep patterns during the day. The pilot in question was NOT a party animal, although enjoyed winding down for a short time with the rest of us in the hotel bar after a days work. This pilot did the correct thing and told me he considered himself unfit for another flight due to fatigue. Ok, so it totally screwed our crew plot and there was no one to replace him so we lost out on a trip (ACMI) and the company lost money. However, it was the RIGHT thing to do and I applaud him for doing it and would not have dreamed of trying to persuade him otherwise. Of course, if we had enough crews in the first place......
Doc C