This does sound like a failing/failed PSU. The most likely issue is failing capacitors in the PSU itself which was common with the CapXcom capacitors used in both PSUs found in the MDD G4s. The two PSUs found in the MDD G4s were either the AcBel 400W or the Samsung 360W depending on configuration.
As for measuring the voltages you will have to have to have it plugged into the motherboard as the PSU has to have a complete circuit for it to provide the voltage. Do note that even though it does have a 24-pin connector to the motherboard it is not a standard ATX and has a different pin layout as well as a special 25V rail for driving ADC displays. I unfortunately do not remember the exact pin layout but to check carefully use a voltmeter between the exposed metal of the case chassis for ground and each individual exposed contact of the 24-pin connector on the backside of the motherboard. Be sure to record your measurements. There are:
* 4x +12V pins
* 1x -12V pin
* 4x +3.3V pins
* 4x +5V pins
* 1x +25V pin
* remainder are ground and 1 power on pin which should all read 0V
If you are comfortable with taking apart the PSU and replacing and soldering in new components you could have a look through [link|https://www.badcaps.net/forum/showthread.php?t=8934|this thread on Bad Caps forum|new_window=true] for more information. Otherwise you could try finding a refurbished unit or having your PSU repaired [link|https://www.ebay.com/itm/REPAIR-SERVICE-Apple-PowerMac-G4-Mirror-Drive-Doors-Power-Supply-MDD-/141030901102?hash=item20d618c56e:g:~RsAAMXQysxSAA61|like offered here on eBay|new_window=true].