The three-prong connector's middle is ground. Each of the other two is connected to the top of a small value resistor in series with the cathode of one of the power tubes to ground.
So, measure the resistance from an outside pin to center pin.
Then, use that value to calculate the voltage for a particular bias current.
If you want to bias each tube at 30ma and the resistance you got was 1.5 ohms, the voltage you want to see across each outside pin to center is V= 0.03 X 1.5 = 45mV.