If you have a digital multimeter on hand, it’s pretty straightforward to test your PSU and rule out power gremlins as the source of your computer problems. Standalone PSU testers are great, and we always have one on hand to get quick results. They can even give you useful values like the Power Good (PG) value that shows you how quickly your PSU comes up to full power—that’s something a multimeter can’t do.
But lots of people have digital multimeters on hand already and don’t have a PSU tester lying around. So while it’s nice to have a PSU tester for those little extra features like the PG value, you can get almost all the same data with a more hands-on approach using a multimeter.
These tests will work fine for the start-up and idle voltages, but won’t be testing the PSU under high load (which is only applicable really when gaming or rendering video files, and the PC fails during those types of activities). I’d also do the inverse of these tests to test all the ground wires are good, i.e. put the red probe in one of the working positive voltage pins, and then use the black probe to test all the ground pins.
See https://www.howtogeek.com/833959/how-to-test-your-computers-psu-with-a-multimeter/