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 was new, the power supplies weren't and I really had no way to isolate the issue between the two. Rather than messing with any of my working systems, I wanted to test the PSUs without having to connect them to a 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, my issue was with the mobo - busted :/
Update: The modtown website appears to now be defunct. A cached copy can be accessed via the wayback machine. Praise the Laud!