Nowhere.
Because the V-in circuitry has no connection to the processor.
You could monitor 3.3volt VCC for a slight drop when the battery is nearly flat.
Or keep track of how long the ESP has been used after a re-charge.
Two external resistors is easier, and they can be of relative high value.
ESP8266 boards use values of 100k-220k. Virtually no drain on the battery.
Leo..