I heard that Neon bulbs can be used in series with certain resistors to measure such as 100, 200, 300 volts. How is this done to catch the leakage volts from higher voltage cable? There are various plastic farmer types that they use on electric fences, but they start at 2000 volts using Neons.

 

Neon bulbs cannot be used for voltage "measurement".
They will fire ON at about 150 volts and maintain about 90 volts when lit.
They are sensitive voltage detectors and can be used as some sort of a crude voltage "regulator" is certain applications.

 

NE2 neon bulbs will fire at less than the 150 volts, and once conducting they will have a voltage drop about 65 volts. One very interesting thing is that the firing voltage can be reduced by external light. I experimented with old neon night lights, and those that would not light up in the dark could be triggered to light with just a flashlight beam. That makes sense because it is the total energy applied that allows the neon to ionize and illuminate.
For a leakage detector you will need a series resistor to limit the current. And yes, they do draw current, and the voltage does vary a bit with the current. And you will need to calibrate your detector to get accurate information.

 

you mean something like this:
https://electricfencecanada.ca/shop...itoring-remotes-testers/fence-volt-indicator/