Assuming you have an atx power supply.
Unplug the main 20pin power connector to the motherboard,leave the HDD, CDR, & FDD conectors fitted.
Get a bent paperclip and short out the green wire pin to the adjacent black wire pin. Plug in the mains chord, and if applicable set the power supply's power switch to on.
If the Powersupply fan spins, listen for the Hard drive spinning up, if they do your power supply is 95% chance of being ok.