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.
Pin 30 is 5v but the power sence and power on or pson pin are still unknown.
I just don't want to see the magic smoke after connecting random pins
Try what I told you, the PSKILL pin (the short one) needs low voltage or gnd connection. The pin you see right below it on the other side of the board is exactly like that).
You already found the pins by measuring the voltage of the whole connector!
The PSON pin is one of the low voltage pins, because its not grounded so it catches some minimal voltage by induction from the board. Try connecting them to ground (guys on the forum recommended doing it through a .5k ohm resistor, and it should turn on. First connect the pskill (short pin) to gnd and see what happens, and then (keeping it connected) go with low voltage bottom pins one by one to GND and the one that turns it on is the one you need.
GL