Hi
There are lots of fancy options some of which are expensive. However I have been very pleased with a simple volt meter, which is connected to our three AGM batteries and placed on a very visible spot in our galley so both my wife and I see and monitor it. It only cost about $10 on Amazon, and has been perfect for over 5 years. It shows battery voltage to one decimal - - ours charge at 14.3, are charged at 13.4 and we allow it to go down to 12.4 and then recharge with engine or shore power.
Our previous batteries lasted 13 years, and are current ones are doing well at 5 years with lots more to go.
Frank