gearlever
Forgive me if you know this but just in case... "acid test" is a colloquialism (vernacular) used in English to describe what test/decider you are using, above all others, to decide the result of something..
e.g. Following a check ride: "Never mind that he flew the instrument rating within the numerical limits, the acid test here is would be you be happy with this individual flying your family from A to B in bad weather?"
Hence the comment in the original post-
The acid test, as always is, if following any training/checking session you would not be confident with the pilot flying your loved ones into some of the demanding destinations, terrain, weather and ATC environments which we operate into, then you have to flag those concerns to Training Management by accurately grading and reporting your observations.
'Acid test' - the meaning and origin of this phrase