To test a G5 PS, you must measure for a trickle voltage on Pin1 . There will be+5Vdc (btwn Pin1 & Pin 23 GND) after plugging in the AC Cord. If there is no trickle , the supply is bad.
(Note: Pin 12 and 24 are empty. Pin 13 is GND. These are the other 4 corners. You probably will have to remove the black cover to expose the wires).
Pin 14 is PWRON. I've never jumpered that to ground to force a power up, but that is the only signal listed that is similiar to the ATX signal used to force a power on.
I doubt that the front panel board is bad, but there is always a first time.
I've seen a failed processor not allow a PM G5 to power on, so nothing in these units surprise me anymore. (You can run 1 processor at a time, but return them to their original slot. Otherwise the thermal calibration will be off and the fans will run fast all th time).