Actually Pozidrive there is at least something that is being looked at as a decent and reliable indicator if someone is too tired to perform certain tasks. That something is how quickly one's eyes/pupils can contract or dilate.
The idea here is that whether on the line of sleepy or truly fatigued the bottom line is impairment. Again, this is being looked at and has by no means been perfected. It does however, show a lot of promise and was not something that sprung from the flying industry but rather after looking into shift work accidents.
I am sure the airline industry would push back violently against such a "test" but I would like to see some of the science progress with it. Impaired is impaired without rest no matter what the reason.
Remember, just something that is being worked at...