For anyone else that stumbles across this in the future: what I ended up doing was setting up a simple voltage divider (https://randomnerdtutorials.com/how-to-level-shift-5v-to-3-3v/) and connecting the 3.3V output as an input to one of the GPIO pins, set to pull down.

When the TTGO board is running on battery, the 5V pin I'm using for input from the external power supply becomes a 5V output, which backfeeds into the 5V rail I'm using to detect external power. To solve this, I put a diode between the external power rail and the 5V pin on the TTGO board. This does result in some voltage loss when operating on external power, so I've been advised to use a Schottky diode instead, but I haven't tried that yet.

So far, so good!