A digital voltmeter is helpful, but its difficult to determine the state of charge from the voltage alone. It can be done at least approximately by measuring the voltage after the battery has rested and compensating for temperature, but its not easy.
As an alternative
out how much you use based on the consumption
of your lights and fans and make sure you recharge well before this consumption
reaches 50% of you battery capacity. If your usage is reasonably constant you may be able to say I need to recharge after X number of nights. If you can recharge earlier and this will be better for the batteries.
2. Install a battery monitor
. These are more than a voltmeter. They count the AHrs in and out of the battery.
Models include the Blue seas VSM, link 10, Nasa BM-1, Victoron BMV600.
The Nasa unit is the cheapest.