Your PSU seems to be a common one and the necessary pins were determined by members of an external forum devoted to radio control enthusiasts found here. The solution seems to have been discovered by trial and error, and the poster claims that shorting pins 31 and 34 (which should be labelled) allows 12V to flow to the rail.
Server power supplies don't tend to have typical connectors like PC power supplies do, but instead have some sort of interconnect system consisting of pins or exposed pads like those seen below (with the resistor and solder omitted, naturally.
Most of the interconnect is taken up by those two large pads on the right which can generally assumed to be 12V and Ground given that the job of PSUs is generally to deliver 12V to the server rack at high current. That doesn't leave many pins left to test and process of elimination can narrow the choice of which pins you should test.
One member of an RC forum compiled a short guide on determining which pins to short if you're going in blind given some assumptions about how the supply works and what it expects. Since these PSUs usually short the DC ground to the chassis ground, all of the ground pins can be determined using a DMM and eliminated. From there, the voltage on the rest of the pins can be referenced to ground and the number of pins to test is decreased substantially. Shorting one or more of those pins to ground through a resistor should likely result in the PSU turning on.
Note that this assumes that the power supply in question requires a simple active-low/high logic voltage to turn on. More complicated supplies or supplies requiring some other control signal will likely not work in this fashion.