SMPS ( Switched Mode Power Supply ) , the power supplier to Motherboard and all other components is the main part of a CPU. How do you know when your computer stops working on a sudden and whether it is the complaint of SMPS or not?
It is very easy to check smps problem. You can check with a piece of power code ( wire) or with multimeter. Use which ever is available with you.
Now, on the 20 or 20+4 pin main ATX power connector, short the pin with GREEN wire (PS-ON) to any adjacent pin with BLACK wire. This process makes the power supply turn ON once plugged to AC main.
Plug the power supply to AC main. PSU fan and connected fan should rotate. You can now test with a multimeter. Here are the voltages:
YELLOW wire= +12VDC
RED wire= +5VDC
Voltage readings must be within +/-5% of above values.
Source: Personal experience with my 11 power supply units ranging from 325W to 700W.