If you have, or can borrow, a multimeter, here's how to test the power supply:

Testing PSU Voltages

ATX Power Supplies

Trigger Power Supply for testing

In depth look at the power supply

Or alternatively you can buy a proper PSU tester, they don't cost much:

PSU Tester

ATX12V PSU Tester

But as Midknyte says, even that doesn't absolutely prove that it's good. I too have seen them pass those tests with flying colours, but still refuse to boot the PC.