To test the PSU, a bare metal paperclip or short length of wire or similar metal conductor is required to electrically short out two adjacent ferrules within the main 20/24 pin motherboard connector from the PSU.
Switch off the PSU.
Remove all PSU wires and connectors from the motherboard. Leave the PSU plugged into the wall socket or UPS, and leave the power supply cables still connected to the PSU.
Connect the paperclip or wire or other metallic conductor across ferrules 16 and 17 of the 20/24 pin ATX main connector. Referring to the image below, the two ferrules are labelled as PS_ON (shown as green color) and COM (shown as black color) right next to PS_ON.ATX 20/24 pin connector diagram
Switch on the PSU.
Then with reference to the diagram and using a digital multimeter, measure the voltages across any two ferrules of the ATX 20/24 pin connector. NOTE: that the negative (black) probe from the digital multimeter should be connected to ANY of the ferrules labelled as "COM". The positive (red) probe from the digital multimeter should be connected to any other ferrule that is NOT labelled as "COM".
Take note of the voltage readings. The readings should match that of the diagram.