I think this guy was asking this question on bitcoin-otc the other day. If it's anything like my seasonic, the larger connector holds more common lines (12v, 5v, 3.3v, ground), and the smaller one holds the minor lines (5v standby, -12v, -5v PWR_OK, PS_ON#). My best bet is to connect a multimeter's probeto a known ground, and start sticking the other probe in the ATX 24 pin area on the power supply. You don't want to dead short random pins with a paper clip because if you dead short the standby 5v toa  ground, you're power supply will go pop, magic smoke.