Measuring extremely high impedance, extremely high voltage sources
Use a variable high voltage power supply to generate a voltage that is about as high as the one you want to measure.

Place a voltmeter between both voltages and regulate the HV power supply until you can't see any difference.

Now both voltages are the same and the voltage source you want to measure isn't loaded. 

The HV power supply either should have a calibrated scale control or its impedance should be low enough to measured its voltage directly.  

        use reading of voltmeter
      to regulate HV power supply
      +----------------------+
      |                      |
      v                                        high impedance
variable HV ----------------(V)--------------- voltage  source
power supply                                   you want to measure