If you haven't already done so, you measure the Plate Voltage coming off pin 8 of the rectifier.  Careful doing so, it's high voltage. 

Then you measure the voltage from Pin 8 of each power tube socket to Ground (red probe on pin 8 of the socket, black probe on a chassis ground), this will be in milliVolts.   1mV=1mA of cathode current thanks to the 1-ohm resistor running from pin 8 of the power tube sockets to ground.

At the top of this page, select your power tube style and enter your plate voltage. 
http://www.webervst.com/tubes1/calcbias.htm

It will tell you what the cathode current should be for ~70% dissipation.  Adjust the bias pot and keep checking cathode current until you get the tubes in the ballpark.