Try to measure with a string of 10 x 10 M
hm: resistors in series,
and have 1 x 1 M
hm: in series too.
So all up you have 11 resistors in series.
At 10 kV you get nearly 1000 volts across each 10 M
hm: resistor.
Across the 1M
hm: resistor you should get about 100 Volts which should be ample to lit a neon indicator ( which usually strikes at about 70 Volts)
Of course have the R's well spaced out on your tester and the 1M
hm: Res. will be at the end of the string with one leg to the ground.
It's not super accurate but it will give you an indication of the voltage level.
Also a neon imposes a negligible burden on the tester. provided it doesn't get too much overvoltage.
The neon striking voltage can be determined with a variable dc supply or mains variac driving a bridge rectifier.
Be cautious with mains wiring during testing of course.
If nothing lights up, reduce the 10M
hm: Res. string to 8 or 6 or less just to get the neon to pulse.
Than work out backwards how to get the fence voltage.
say 70 volts across 1M. and 10 x 10 M Res. about 7000 Volts makes 7700 Volts.
All Resistors rated at 1 Watts