Measuring Lithium ion cell voltage used to power Arduino via a boost module
I'm the author of a library called BatterySense and you can use it to achieve your goal. In the repository Readme you can also find wiring diagrams and usage examples, but to give you a rough idea...


Verify the negative side of your battery has continuity to the GND pin of your Arduino: you need to identify if the boost module is messing the common ground, but if you have continuity then you are fine
Add a connection from the battery positive terminal going into one of the Arduino analog pins
Use the BatterySense library to get the battery voltage and percentage level