Actual 780 supplies have Built In Self Test Button. BIST.
The only one without this is the USFF.
The LED on the back does not mean that the power supply is good.
The BIST (Built in Self test) is what that LED and button are for.
Power Supply Self Diagnostic Features
Some Dell computers have power supplies that include a self-diagnostic feature. The test button for the power supply is visible on the back of the system. This LED is not the same as the flea power LED on the motherboard (which also only says +5VSB is working NOT the Power supply is OK). Press and hold the self-diagnostic switch on the power supply -
* If the LED stays green, the supply is functioning
* If the LED stays off, get the power supply replaced
well i have a power supply tester but for bigger power supply connector is there a way to test them withn an adapter plug to it for small 20 pins connector
its 24 pins and yes there is an adapter but not for tester its to convert reguar psu socket to normal for 780/980 tower