Fault finding is basically a process of elimination, it's not based on or around educated guesses... especially on installations you have little knowledge of.
Being methodical using the correct test equipment and understanding/interpreting what that test equipment is telling you, will always identify where the fault lies, or at the very least, localise as to where to fault will be (as in an inaccessible length of cable, say under floorboards etc...)
Fault finding covers a vast area, from basic fixed wiring, through to component level of plant panels and it's control wiring and everything inbetween...
Something i used to really enjoy during my time in the industrial sector. Those were the days of massive panels, full of relays, latch relays, contactors and just about everything else you can think of!!! lol!!