boredcounter,
You cannot be expected to know if a crew member is knackered or not. This is where the system falls down.
Said crew member will 'probably' keep quiet, as per tilli's and hugmonster's posts and of course the airline is 'ignorant' to the fact that some of their crew's could well be fatigued for whatever reason.
It is a very sad state of affairs in the 21st Century if a crew member feels they will be 'sh@fted' if they inform the Company that they are fatigued. Before I get shot, yes I know there are unscrupulous operators out there!