Disclaimer: Doing this can destroy your power supply.
A month or so ago, my desktop computer died. It wouldn’t turn on, after no symptoms of problems. My first guess was the power supply. So, I replaced it.
Nothing.
Next guess was the motherboard, so, I replaced it.
Nothing.
At this point, I didn’t want to spend anymore money if I didn’t know for sure what was wrong. So, I decided to test the power supply. Following this guide I shorted out the green wire (PS-ON) to a black ground with a paper clip while the power supply was off. I then plugged in the power supply until the fan started to spin, and quickly unplugged it.
Now I know my power supply works, if anyone knows how to test a motherboard or processor, let me know.