How you can solve a problem or find out on a dead pc if the problem relies on motherboard and not on processor or vice versa

Question

hi.
I am working myself with computers ,  so there is one thing in my mind I wanted to be sure,which I couldn't find an answer for it!
because I had before totally black screen and the problem was damaged processor.
I am wondering how I can be sure as the last option when I get a black screen and there is no lihgts at all no any LED is lighting.totally dead pc.
how  you can be sure then that the problem relies on processor and not on motherboard or vice versa the problem relies on motherboard and not on processor
  because I can't say to the customer lets buy first a processor if it is the cause of dead PC and if not then we buy motherboard. this sounds not wise.
so I experimented first with my latop ,I pulled out the processor to see if I can see any LED light or any movement on the motherboard to be sure that motherboard giving me some clue that it is healty. I gave some light heat to de fan to let it turn ,because 
it is working with heat sensors then I know that the motherboard is healty. but it didn't work. 
I did the same with my desktop computer and the result is the same no any lights and no any movement on the motherboard . so now can some one tell me how you can distuingish as a last result which of this 2 components is causing dead PC. is there any
tool to test the motherboard or how asus,dell ect. they solve such a problem if the customer send to them a dead pc.
I know that the power supply also can cause dead pc but I know how to solve a problem  with dead power supply. and I want to keep away power supply from this question. please do not tell me that  Idon't have to pull out processor
because I am a technician and both of my computers are doing very good after this experiment.
thanks
johan
h.david

Answer 1

Usually if you have some kind of hardware failure there is some beeping during POST or most motherboards now have LED indicators to produce and error message based on the type of failure
So if its bad memory, not place properly, mismatched, processor not inserted properly, mismatched voltage or voltage connector not present etc it beeps or generates the error id.
Power supplies can be tested for failure. There are some walk throughs for testing just them with a switch, paperclip or a jumper (I'd suggest not doing this if you are not familiar with the dangers of electricity).
Memory can be tested with memory diagnostics programs like Memtest+
Processors can overheat if the proper precautions have not been taken usually you will get a POST beep or error code for that.
If the motherboard has no response then do the basics first:
Check power connectors and power supply. Once you determine that is not the case move on to other items like graphics cards in all the way or memory.