First you need to know the peak voltage of the fence charger and the maximum input voltage of the Fluke. Unless you have a genuine technical need to know the exact voltage, a series string of neon bulbs (NE-2 or the like) needs about 90 volts per bulb to light it. For 2000 volts, that's 22 bulbs in series with a 100K resistor.
If you just need verification of voltage present, an NE-2 with a 2.2 megohm resistor will show voltage present, but so will a NE-2 held by the leads and the glass of the bulb touched to a fence wire. Best to have the resistor unless you're very careful with how close your fingers and the wire get to the fence ;-)