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.
DPS-1200FB A(12V/100A) which I managed to turn on by shorting two pads on the output connector. If you upload more information and an image, I can check if the connectors look similar. – jippie Aug 20 '15 at 15:35