Check incoming power level on Photon

I would like to be able to monitor the incoming power level when using a Photon (with onboard voltage regulator) and signal when the voltage is getting low? Is there any firmware access to the voltage level?

If not, I am considering tapping into the 5V USB power and connecting an analog pin to the line between two 3.3K resistors; similar to the circuit drawing shown here

No there is nothing built into the photon to do this but you can add a INA219 chip to measure the supply voltage and current if you want.

You would then be able to read the voltage like they are doing in the video below using that exact chip.