I had parts for this build sitting around forever, so it's a little dated.
Intel i5 2500k
Asus P8P67LE B3 Revision
Asus GF GTX460
G Skill Sniper Series 2x4GB (I forget the speed, I think 1600)
Rest doesn't really matter, I suppose.
When I turn on the power supply, the motherboard light is solid green. I have tried to start it both by using the power switch and by shorting the power switch pins. I then tried to test the power supply by pulling the 24 pin connector and shorting it with a paper clip. The power supply turns on, LEDs and fan working.
EDIT: Are case fans enough to test the power supply with load? I plugged in all 4 and they work just fine when shorting the 24 pin connector.
I've read through the troubleshooting guide and have gone through most of those steps already, except using a multimeter as I don't have one right now.
Is there a good way to narrow down the problem? Right now, I am guessing either the PSU or the motherboard.
You're not really going to get much of a response because we've already done a lot of work creating a troubleshooting checklist for this exact type of problem. I know you said you read through a troubleshooting guide and did "most" of the steps. Perform EVERY step in the checklist linked in my signature and let us know how it goes.