Basically the shorter pins on the tabs are the power-on pins because they have to be the last ones to turn on when you slide in the PS hot.
The switch connects with a 100 ohm series resistor on pins 1 and 4 of the power supply, on the component side of the circuit board.
Plug in a power cord in the PS, and measure voltage with a multimeter probe on the (+) and (-) tabs.
Some guys like to put a switch on it but that's hackers that are using the PS for something else.
You would need a header socket or very small alligator clips to get on the small 1 and 4 pins without soldering.
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