I'll ignore most of the sarcasm... but I will say like everything in life, there are good quality parts and cheep ones. Good ones can fail just like the cheep ones but are less likely too.
Diagnosing failures also like everything else out there just takes time and experience. There are no hard and fast rules, no most likely it was this or that. It it only something learned over time. It sounds like it's time you start the process but do not expect it to be a quick one... learning isn't a quick thing, it takes time.
You already did at least some of the preflight tests... I didn't see you list a motor run up ( while holding the airplane by the tail typically by standing in front of the elevator ). Still... that is always where you start and make note of anything out of the ordinary.
Next, after an incident... you repeat the process. Are there any loose control rods, surfaces. Do all of the servos still work, does the throttle still work. You go back through and check each and every system trying to see if there are any clues there or not. Next you would inspect all wiring connections to see if anything there may indicate a possible cause.
What you sort of described... could have been a problem with an aileron or rudder depending on how it moved in the air. Ailerons would affect roll turning an airplane, rudder the yaw also turning an airplane. I'll mention the elevator would affect the pitch but your description doesn't seem to indicate that but I didn't see the incident happen either.
If some of your parts now are not working... you would use a process of elimination and just replace one piece in the chain of the non-working part. In the end, you would need to test each one at a time and separate from each other.
Sorry to hear about the crash whatever the cause or reason. I'm sort of amazed you hadn't had them before now.... but it does show why the statement of " crashing, it isn't a matter of IF but WHEN it will happen"