The simplest way to check SMPS (PC Power Supply Unit)

ATX power supply connector
ATX computer power supply with connectors for ...

Quick PSU test:

Attach a fan on one of the 4 pin molex. On the 20 or 24 pin ATX power connector, short the pin that has the GREEN wire with any adjacent pin that has the BLACK wire. Attached fan and PSU fan should spin. You can test the +3.3V and +5V rails with a multimeter.
Pins with the Orange wire carry +3.3V while RED carry +5V. YELLOW carry 12V.