> Some other fence controllers (power supplies) have electronics and
> DO pulse, but not my cheap model.

In that case then you really are measuring plain AC. If you assume it's
660VAC, you could use a standard red LED, Vf = 1.7, I = 5mA

R = (E - Vf) * 1000/ImA

R = (660 - 1.7) * 1000/5

R = 658.3 * 200

R = 131.660k

Put a reverse parallel diode (1N914) across the LED. Resistor will need
to withstand 660VAC (don't know whether you're measuring pk-pk or
RMS) and would be better made of several smaller R in series. General
purpose Rs are usually 200V rated, but err on the cautious side. If the
LED is dim you could reduce R in increments to get the brightness you
want. Rather depends on the specs of the LED

Alternatively a dropping capacitor to limit current (with suitable voltage
rating, eg 1kV). 1uF will pass approx 70mA, so 68nF will allow about
5mA through. Some examples and principles of AC-powered LEDs here

http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm

http://www.discovercircuits.com/H-Corner/AC-Powered.htm

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