Diagnostics of a faulty PSU?

Hi!
A faulty PSU - your PC crashes, does not start, crashes immediately after you put load on it (because the PSU can not deliver, so the component like GPU or CPU fails).
Also when the single lines drop too much, your PC will crash.

If you just want to check the PSU without a PC-system (in order to save the system because you don´t know if it includes safety functions) you need some test station. When you want to do it correctly.
CHROMA stations are the best but you pay lots of thousand of euros.
You can find used machines like from FAST for example for just a few thousand euros.

As private person, this normally is not necessary. Buy some good multimeter to check the lines and create a benchmark form you will use to put different loads on the PSU.
To check the safety functions, you have to open the PSU and check for the safety chip, also for the MOV.

But only do this when you are an experienced educated eletrician.