Well, first of all, check the LED on the motherboard. If there's power coming from the PSU, the LED should lit.
Sort of, but not exactly.
The "sort of" part ... The motherboard LED is powered by the small, always on 5 volt standby power supply. It provides initial startup voltages to the rest of the PSU.
The "not exactly" part ... It has nothing to do with the main section of the PSU that provides operating power to the computer.