The two things that make me sweat when looking for problems in a computer are the power supply and the motherboard.is there a way to eliminate /isolate these problems so that you don't mistake a power supply fault to be a mother board fault display as in the case of a blank display where everything else is okay and you are left with two suspects that is the power supply and the motherboard?

Power supplies can be field checked with a simple tester (Example: COOLMAX PS-228) or replaced with a known good supply.

A POST (Power-On Self Test) card (Example: StarTech PCI Post Diagnostics Test Card) can be installed to see where POST is hanging. Motherboard components can also be visually inspected for signs of failure. Leaking and vented capacitors have been an issue last few years.

Power supply repair tips:
Power supply repair tips: http://www.smpstroubleshooting.com/