the green wire test ? -> https://www.google.com/search?q=green+wire+test+psu&ie=utf-8&oe=utf-8
Others use a volt meter
Digger
I have a hp pavilion slimline s5500 series and when I plug the power cable in the green light is on. But soon as i hit the power button the fans spin and the light dies. Can anyone help me? Do I need another power supply or is the mother board connections bad.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic