Even more simply, I'd use an electric field probe and look for nearby AC. The detector frequency response doesn't need to be very specific at all, continuous and infrequent interference can be excluded statistically, and the signal of interest is more or less periodic.
A suitable input circuit would be a capacitance plate near the fence (doesn't have to be touching it, you're picking up the EMI and bounce that the scope was reading directly), a half-wave voltage doubler circuit using small-signal schottky e.g. BAS70, and an RC filter to remove RF, obtaining a simple envelope of detected RF. A clamp diode (to VDD) should probably be added, and then it's safe to go right into the ADC input. A conversion every so often, clustered near where the next pulse is anticipated, would suffice, minimizing current consumption; solar panels can furnish idle power, and some means of transmitting the status to a central collection site would be needed (probably not wired, but RF transmitters may greatly increase power consumption).
The general insight applied here is that, sense dividers don't need to be resistors, and often are worse when you do -- or at least harder to clean up. A capacitor divider works perfectly fine for AC, as long as the load resistance (here, the detector) is large enough not to mind (i.e., resistance sets the LF cutoff).
Note that this doesn't need to be done very close by, at all; indeed we could opt for an RF receiver, perhaps tuned to a resonant frequency of the fence system -- this would have to be probed, but could provide some gain, and rejection of interference, in the process. Such an RF amp or chain might be built with BJTs, maybe a JFET at the front end; current draw of fractional mA is reasonable, I think, while getting a noise floor not far above atmospheric noise (and not requiring much of an antenna). It might even suffice to listen for blips on an AM radio, no custom hardware required. A directional loop might be able to provide specificity, in case there are multiple fence chargers around the property.
Tim