Is there a way to determine the battery level on MG100 using Canvas?

Yes, battery level can be read on P0.02_AIN0 via the ADC machine class in Python.

https://lairdcp.github.io/canvas_python_docs/machine.html#machine.ADC

 There is also a python sample provided that measures and returns the battery voltage level.

 https://github.com/LairdCP/canvas_python_samples/blob/main/apps/mg100/adc/main.py