Uncle Fred,
validated tests for quantifying fatigue, though possible, are difficult to undertake, especially in view of multiple confounders. It is easy that the regulators and airlines redefine FDTL (EASA already has a draft notification in place) and the stakeholders, the pilots unions, are chosen to be equal partners.
At the individual level, each pilot has to be accountable but for him-/herself, including following hygiene factors like abstaining/acceptable
consumption of alcohol, adequate rest/sleep and willingness to owe it up, when not ship shape.
Or else, we may have
crash due to probable sleep inertia, or
deplorable acts by airline managements, without worrying about the outcome or aviation safety.