Most end users do not have the tools to properly check a power supply. You need load resistors (4 or 5 100W resistors), a watt meter, and a volt meter.
The only other way for an end user to test a power supply is to swap it out. You can look at the system BIOS and check what is shows the voltage levels are for the various rails, but that is only good for power supplies that are really dieing an ugly death.