ALL that counts is the suitability of the flight deck crew for the job they are required to do. Fly the aircraft from A to B.
Are they mentally fit for purpose?
Judging from the medical experts above, in that there is no "test" for this, the best you can hope for is "maybe"?
Are they physically fit for purpose?
Once per year they are assessed, and certified as "fit for purpose". In between times it is "self assessed". As detailed elsewhere, given the implications of calling in sick in some airlines' HR policies, it would appear quite often "not". And as also detailed elsewhere, frequently fatigued to the same extent as having imbibed in one or 2 alcoholic drinks
Are they trained, and current, to meet any scenario that faces them on any particular flight?
No - no pilot has ever been trained to meet "any scenario". Twice per year in the Sim, and once per year or 2 via Route Check they are assessed in a program set by the regulator.
To the point where there is zero risk of 'pilot error', whatever?
Never
Only then will the travelling, paying public be reassured into placing their trust, lives and faith into their hands.
Best they get a train timetable then
It would appear that hose who control and enforce such matters are still falling short of their responsibilities.
I personally think the regulators do a pretty good job, but not perfect. But you seem to have some expectation of their responsibilities that far exceeds realism
