I think you could also just monitor the voltage directly using an analog input pin.
How much power do you consider a drain? any voltage divider or reference diode like the TIL431 requires power. The TIL431 is an adjustable Zener diode and does require 400 uA as a minimum cathode current for proper operation. It and a pnp transistor could make a switch for flagging when the voltage to the device was above a "Set Point" but it would require about 1 mA for proper operation when used to measure the primary power source and there's a way with 2 transistors to enable the device that isn't very complicated and draws no current until enabled. 3 transistors 2 pnp's and an npn and about 4 or 5 resistors total maybe a .1uF cap on the '431 for stability as there is a LOT of current gain in that device. About 100 uA to control up to 100 mA of Zener current through the diode.Doc