If the 20/24 Pin power connector is not attached and you only want to test if the psu Works. Take the 20/24 Pin connector, then take a paper clip and connect a black and green wire together using the paper clip.
If the Power supply is good it should turn on. But don't keep it on too long. If it does not turn on,
1. Its no plugged in
2. The on/off switch in back is switched to off
3. Or its a DOA and should be RMAed.