Modifying 12V PSU, how do I identify the +12V Return Sense?
I would suggest:


Feeding a low voltage into the 12V output and probing for points that are always equal to (or at a fraction of) the applied voltage.
Finding how these points are connected to the output; they will likely be connected via a divider.
Somehow tweaking the ratio (i.e. by cutting the line and inserting another divider) and seeing how the circuit reacts. It should bump up the output voltage as you divide the feedback.