How to tell if non-booting PPC G5 is due to Power Supply or System bd

I have a late 2005 G5 that won’t boot. How can I tell if it’s due to the power supply or the system board?

When it was working recently I reset the PRAM, ...that gave me back my chime...

I pushed that tiny button under the RAM and that helped start it once or twice. I replaced the PRAM battery (a different type on this machine...shaped like a nickel and made by Panasonic).

I get a yellow light inside, a white one showing it’s on and a red one lighting up inside for a second, but no booting.