There is no internal ADC channel that can monitor the power supply. In all my designs I put a voltage divider on each supply that I want to monitor, then connect that to a spare ADC input. If you are sensitive to wasted battery power, you could switch the divider in with a FET or use large value resistors with a bypass capacitor, say 0.1uF or so. In a system that is not limited to using a high impedance divider, it is also practical to connect the same monitor to a comparator input. You can use that comparator function to get an immediate notice when power is going down.
