Remember one of the first instruments we spoke of? Ears? It is vital to allow the client to speak, and for you to listen. "It only happens on hot days" could be because an air conditioning system only works when the air temperature is above 25°C, or, an overhead line sags sufficiently to touch a tree branch grown into its space. Conversely, only happening in winter could very well be related to a central heating system that has an electrical malfunction.
Using the principle "rule things out, one-by-one" will, in the long run, save time. Rather have too much data on the file than too little (who knows, it may prove prudent in the future - especially with a problematic client). Forming the complete picture with evidence is a far better approach than using assumption to ascertain a reason for bad power or fault conditions. This is especially true if the complainant is making a claim against an electricity supplier.
Added to this, don't forget to make a list of equipment found on site. Again, don't treat anything as without fault until proven such.
In one investigation the complaint was flicker caused by an American style fridge. Although the problem of high start currents of a 60Hz motor running at 50Hz (usually over 20A) was recognised, this was not the real cause of the flicker. When talking to the client and listening to how everything from an iron and kettle through to the dishwasher caused the lights to dip, focus was turned from the fridge to the house wiring which was later proved to be the major contributing factor as it was far too light for the style of premises.
This section comes with a small word of warning. Don't let anything you find cloud your judgement or, worse still, lead you up the wrong path. An example of this would be transients. These are part of everyday life but concentrating on them may find you missing the real fault completely, for example high leakage currents.
A final word of caution. Don't fall into the trap of grasping at straws. It is not unheard of for less experienced individuals to hear of a type of fault - the highly technical one full of buzz words is most attractive - and to then try to work this into their situation in a vain but dangerous attempt to save face with a client.