I had an issue today where I was stuck with a non-functioning board and 3 power supplies. I couldn't get any response from the board regardless of which power supply unit I was using. However, while the board is new, the power supplies weren't and I really had no way to isolate the issue between the two. Rather than messing with my existing systems, I wanted to test the power supplies without having to connect them to the motherboard. The ever-reliable Google gods pointed me to this modtown article which details how exactly to go about getting this done.
Simple and effective ...
P.S. To ease any itchy brain cells out there, the issue was with the mobo - busted :/