Alot of "Diagnostics" comes from experience, but every now and then you'll be in a hurry and throw parts. LOL! I was in the auto business and even went to trade school for it. I remember this basic principle, "there are four things that will make an engine not run, no fuel, no spark, timing, and mechanical failure" such as a jumped timing chain, or head gasket blown etc. I would check fuel with a can of carb cleaner, on carb or FI engines spray a blast or tree down the throat ,crank it if it fires and runs 10 seconds you have spark...that test takes 10 minutes max. That will also tell you if the timing is good if it runs smooth for those 10 seconds. Then you know its fuel delivery and you focus on that. if the car starts and backfires and runs like crap, its timing or mechanical failure. than you go from there. It doesn't take al day to do diagnostics, and you basically "follow your nose" like a detective. You don't get into tearing everything apart, you focus on an "area" to take things apart, if you think its valvetrain ,you pull the rocker covers and look etc.
