Capacitive AC voltage detection
I ended up going with a circuit based on the design here. With my modifications, it seems to work well, and does not pick up other RF emissions as false positives.



simulate this circuit &ndash; Schematic created using CircuitLab
The antenna is a 4-inch length of solid 22 gauge wire. Out is high until voltage is detected. D2/C1/R1 filter out the 60Hz pulses from the 4011 output, so that the output stays constant.