I guess it's just a natural thing when a vehicle is acting up to just throw some parts at it... or rev it up and see if that fixes it.
Over the last 50+ years of doing my own wrenching on all makes and models I've discovered a few things to be fairly common.
It seems to always help to start with the simplest explanations first...
Do I have fuel?
Do I have fire?
Are the fuel and fire getting where they're supposed to, when they're supposed to?
Knowing your vehicle and experience can help with diagnosing problems, along with common sense to eliminate wasting time and money... vehicle specific sites and searches may confirm your suspicious notions.
Here is an example...
I was driving my 85 k10 pickup home the other night... turned out of a lease road, on to a county road gave it some gas and it died.... didn't cut out miss... just died. Out of gas? Fuel pump? Fuel filter?
bad connection,?
nope I KNOW this vehicle and instantly knew the ignition module took a crap....i wasted no time on any other possible issues because of past experience and knowing my vehicle.
new ignition module... bam fired right up...
I've also found that many times its something simple llike a ground issue, if its electrical or a fuse.
Fuel is usually filter, pump, or regulator...
battery terminals are a common failure that can cause weird problems...
when all else fails check the muffler bearings and turn signal fluids...

 

I always like the "Test, don't guess" method to diagnosis. I hate to throw parts at a problem, especially if the old parts are perfectly good OEM parts and the replacement parts are just the "parts store special" of questionable quality. It is possible that you actually insert another problem that way.

If I come across a problem, I want to make sure I understand the system I am trying to diagnose and then get a plan together for the most logical plan for testing.

Past experience can usually short-cut a diagnosis by letting you get to the most likely failure prone part first. Just be careful though as just because something is common to fail does not mean that particular part is the problem you have in your current situation.

If a fuse is found to have blown, I would want to know why the fuse blew because fuses generally do not "open" on their own. Usually there is another problem somewhere in the circuit that the fuse is protecting.

Wiring diagrams, and multimeters (and test lights) are very helpful when tracking down electrical problems. If many electrical functions stop working at the same time, usually trying to diagnose the simplest circuit of all the circuits that went bad can lead you right to the common problem.

For modern cars (1996-present), a scan tool that both gives codes and live data is pretty much required. A bi-directional scan tool is even better. Since modern cars are computer controlled, you want to know what the computer is seeing and make sure the computer inputs are working as they should.

Visual inspection can also lead you right to the problem. As RAD4Runner says, what you see, hear feel, smell (hopefully not taste) can go a long way in figuring out a problem.