It never fails to amaze when a working donk becomes a bag of scrap metal when any investigation into its interior is carried out.
Engine failures do however happen when airbourne.
Devil and the deep blue.
In answer to the question how do you tell, I am afraid the answer is you probably can't.
Since the BS 9000 quality systems don't necessarily apply to maintenance organisations, who are licenced specially by the CAA, I do not believe any systems exist to measure customer satisfaction or cost effective workmanship.
If such a system did exist, I do not think the industry would let the results become public.