One way to determine the battery status of a flashlight is to measure the light output. First measure and record the LUX output of the flashlight with fresh batteries. At a later time, you can measure the LUX output and compare it to the recorded value.
A simple LUX meter can be assembled from an Arduino and a BH1750 Digital Light Meter module (~$3-$4 on eBay) which measures light and outputs a digital value in LUX. The measurement can be displayed with an OLED screen connected to the Arduino.
