Usually the fat tabs are 12v and ground, because they will hold the greatest load. Looks like you have 4 pairs of 12v+ground (left/right and top/bottom). There is likely also a power sense that is supposed to be attached to 12v, and a power on pin that, when grounded, turn on the power supply when combined with the sense > 12v. One of the small pins should also be a standby 5v that always puts out 5v.