battery indicator

I was simply wandering if there was any way of measuring how much power is left in your lipo battery, even if it means landing the plane to have a look. Also, what do you guys charge your batteries off. So far I have stuck to a car battery - is this the right thing?

I bought a ORANGERX TX6I Radio with a ORANGERX R618XL 6CH receiver. I have yet to get to my maiden flight. But one thing I would really like on my plane is a module that transmits the battery voltage to my radio. I saw something about this on the radio and was wondering what the name is of such...