ATX is Advanced Technology Extended. You can use ATX power supply not only for computer peripheral (Mainboard, harddisk, CD/DVD Rom) but you can use for other electronic device too, depend on voltage and current of it's device.
The ATX Power Supply has DC (direct current) output : +3.3 V, +5 V, +12 V, -5V and -12V.
To Switch On the ATX Power Supply without connect to Motherboard
Find out the Green wire
Find out the Black wire (many black wires, use only one of them)
Connect (make short circuit) between these green and black cable. you can use on off switch or use cable jumper or you can do manually by cut and joining these cables.