Arduino MKR GSM 1400: Can I tell from code if the power has tripped
VBATT/2.4B is used for measuring the batter voltage using the internal ADC of the MCU. It can't really tell if the battery is being used or not - except by maybe monitoring it over time and see if the voltage goes up or down.
However the BQ24195 is where all your hope lies.  It has a status register that can tell you all sorts of things.
For example it can tell you if there is incoming power, and what that incoming power is coming from (a USB host, a USB power adaptor, or operating in OTG mode). It can also tell you what the battery is doing - if it's charging, or not for example.
A quick google search throws up this library specifically for the purpose.