i can't find the picture guide i had bookmarked but its not hard. John here has some links.
in a nutshell, pick yourself up a $20 dmm if you don't have one and open up the case.
put the black lead from the dmm into a ground wire (black) and than probe the line you want to test-
yellow= 12v
red= 5v
orange= 3.3v
you can get the 12 and 5 from a 4 pin molex and the orange from either an sata connector or the 20/24 pin connector.
if you have dual 12v rails, test the other 12v rail off the 4 pin atx connector.
test them at idle and then i usually fire up prime, rthdribl and rip a dvd to get some load on it.