Detecting HI-Z state of charge controller
The best way to measure the Hi-Z state is to hook up a voltage divider to the output. When the output is not being driven, the voltage will settle at the potential set by the voltage divider, which provides you with a third voltage to measure.

Distinguishing these three voltages using transistors is tricky, and probably not worth it - using an MCU, or failing that, window comparators, is a better solution.