I better understand your question now. I would like to rephrase the first statement that I made above. "...anytime you suspect that there is a problem." What are the signs of a problem? 1. Smoke 2. Fire 3. equipment doesn't work 4. equipment doesn't work properly 5. someone has been shocked (hopefully not killed).
I think the problem is that people are going to act in accordance with their experiences in life, and according to their perception of the events taking place. If experience tells them that "nothing is wrong", you'll be hard pressed to convince them that they have a major electrical problem no matter what warning signs are present (i.e. someone lying dead on the floor)."Perception is life's cruelist form of reality", someone once said. If someone perceives that there is a problem that they can't handle, you can talk 'til your blue in the face, and they won't be satisfied until you charge them a $200.00 service charge, and repair something regardless if it's broken, or not.
The short list of warning signs above should generally cover most applications. I'm sure that many electricians could easily add to the list, but my intent is to paint a broad picture. More sofisticated equipment would, I'm sure, call for more specific "warning signs".
Remember this "Proper electric today, keeps "The Watt Doctor" away."