An appliance repairman will have many different diagnostic tools. Unfortunately, diagnostic tools only go so far. Some specifics in your case:
My microwave (built-in range) that doesn't always heat to the same level.
There are a lot of things that can affect how effectively a microwave oven heats food, beyond any actual problems with the appliance:
- Starting temperature of the food
- Moisture content
- Material (glass vs. ceramic vs. plastic) and shape of dish holding the food
- Oven setting (high/medium/low/etc.) - which may seem obvious, but is actually quite different from a conventional oven.
In general, a consumer microwave oven only heats at one level. It varies the time of heating - e.g., medium might heat 2/3 of the time and low 1/3 of the time. While on a conventional electric oven you will typically have 2 (sometimes more) elements, so an intermittent failure of one element might leave the oven working better at some times than at others, I don't know of a similar failure mode for a microwave oven.
If you are heating based on time, the many different variables will result in different results. I would only suspect an actual appliance failure if it sometimes does not heat at all, which could happen if there is an intermittent problem that prevents the magnetron from working.
As far as diagnostic tools, a repairman may have (probably should have) a leak detector. But I don't know of a tool that tests "how well is the microwave oven heating".
My oven that sometimes pre-heats, sometimes doesn't. Sometimes it starts at 100 and displays incrementally what temperature it is, sometimes it starts at 105-350 (depending on what I set it to) or, if I set it to above 350 it stays at 350 and doesn't tell me when I'm at 400.
Different models (or more generally, different manufacturers/product lines) handle temperature displays, preheating, etc. differently. Some show a temperature (Low->100...350) while preheating, some just show the set temperature (350) and beep or show an icon to indicate the temperature has been reached. There may be variations also based on the particular mode (regular, convection, timed). No real diagnostic tool for the functionality - more a matter of methodically working through the different modes to make sure everything works or to determine what is not working. The one tool would be a good thermometer to verify temperatures. Anything capable of oven-like temperatures will do, but the professional solution is an infrared thermometer like: