If the charger decides to charge the battery, then the battery is "low on voltage".
If you want the Arduino to monitor the battery voltage, you can read that directly with an analog input.
Note: unless your battery already has an overdischarge protection PCB, be sure to use the version of the TP4056 module that has one built in: Micro USB Li-Ion Battery Charger Module TP4056 5V 1A Protection 18650 - US SHIP | eBay