There is no single way to test the motherboard, they have hundreds of components to test. Easiest thing to check is for bad capacitors, happens a lot. They'd be puffy or actually burst at the tops.
Aside from that, you'd have to basically learn how the motherboards work, what part should have what voltage, resistance, etc..., and start checking them. Plus they are usually built in layers, have different other chips on them aside from the CPU. Good project to play with if you really want to do this, but don't except someone to say "just hook up wire A to part 1, wire B to part 2, if you get no reading it's bad".
Failures could be from he power supply also, or the video card. Those two often cause computer issues.
I kinda suspected that there would'nt be a simple way to do this,like touch both ends of the MB with the Multimeter prods,and if the current flowed thru,then we could assume she was ok.What a dreamer am I.However,as you suggest I will check the capacitors,power supply and Video card (I have replacements),Thanks again for your help.