The most reliable way to check the state of charge for your battery is with a multimeter. This guide will show you how to measure that.
The first step is to turn on your multimeter and make sure you are reading DC VOLTS - see the following image for reference.
Voltage is measured from the DISCHARGE PORT on the battery - this will be circled for each model. You will need to remove your battery from the bike to do this.
Voltage is read from the two large holes with + and - to the left of them in the below image.
It doesn't matter which probe goes in which hole - the number provided by the multimeter is the important part
Voltage is measured from the two circled ports
Range Extender Battery
Before reading the voltage on this battery make sure to turn on the power switch on the battery like in the following image:
Read the voltage from the circled ports
Read voltage from the circled ports: