How do I know PC power supply failure?

The story,

The power distribution in many industrial areal aren't so consistent. Therefore we frequently have dead PC power supply.

So if we cannot turn on the PC, how do we know if it is the power supply failure and not the motherboard?

Here's a simple solution,

1. Open the PC casing, and look for a small LED green light (mostly beside the power connection to the motherboard. 
2. This is a power standby indicator that signal there is power into the motherboard.
3. If there is green light when AC power from wall socket is turned on, this signal that the power supply is OK and vice versa (picture below).
4. If there is no light, change a new power supply.
5. If after change the power supply also no light, then it's the motherboard problem.

Additional note :

• Don't attempt the repair the power supply unless you are a trained electrical technician. The power supply is so cheap that it's not worth to repair even you are a technician or engineer.
• There are still charges in the capacitor that might kill you even it's not plugged in. Don't pass these hazardous things to children or anyone else. Stay safe.
• To keep your power supply healthy and lasting, I suggest you use a AVR (automatic voltage regulator  or UPS (uninterruptible power supply) regulate the AC input power.