Unplug the computer from the wall
Take your computer's case cover off
If you're not used to this, don't worry. Its not a big deal at all.
Remove the main connector from the motherboard
This is slightly scarier but only very slightly. There is usually a small clasp on one side that you have to hold open while removing this from your motherboard.
Plug in a case fan or a hard drive
Some PSU's require at least a small load for this test to work.
Bend the paperclip into a U shape
Bridge the paperclip between the green wire and a ground (usually black) wire
If your PSU is a corsair like mine, then any black wire is a ground wire. This is not dangerous at all, its just basically telling the PSU to power up as soon as its turned on.
Plug in and power on the PSU
If the PSU fan powers on and starts spinning, then your PSU is most likely good! In my case, my PSU didn't budge so I know its busted and I'll be ordering a replacement.