Actually, 'troubleshooting' is what you do when the customer has already had someone less competent than you try to 'fix' something. Do it well, and the customer will become a believer.... and might even stop trying short-cuts.
My personal rule is: Two 'best guesses,' and then it's time to get systematic.
"Get systematic" means to start over, from the very beginning, with no assuptions.
Step #1 is to define exactly what the problem is. A "lights out" problem was quickly diagnosed, when I saw that one fixture would operate if one circuit was on, but not the other.
Part of "Step #1" is reconnaisance; that is, looking the place over, and seeing how the bits connect to each other.
For example, a "receptacle no longer working" call was solved when I discovered that an entire wall of receptacles had never been connected to the panel. OOPS!
Step #2 is to localise the problem. Depending on the circumstances, you can either "follow the power from source to solution," or "divide by halves."
Step #3 is to be curious. I recently solved a problem only because I was curious as to how two red wires at the panel became two green wires at the appliance.
