bad motherboard or cpu?

Here is the situation. For a few months one of my computers has been randomly rebooting every few days. Today it shut down and it won't even power up at all. I am quite sure it is the motherboard or cpu because I have tested the power supply and all the other parts in other computers. I can't test the cpu or motherboard because I have nothing to swap them with. All the capacitors look fine on the motherboard and the cpu looks fine as well. Help me /comp/

if not the power supply, then the motherboard right where the power connects.  overheating power supply usually causes that, check your fans before using them in other computers.

can also be RAM, CPU, any of the cards, etc.

hence why they have testing centers...