Measuring the state-of-charge of a lead-acid battery is non-trivial.
The easiest way is to use a voltmeter. You'll need a digital meter with 3 1/2 digits of precision. Let the battery rest, disconnected, for 24 hours. Then measure the voltage across the terminals. According to http://www.phrannie.org/battery.html, anything over 12.60V is at least 85% charged. Maintenance free, gel, AGM, etc. batteries have different voltages.
Batteries vary, so you'll want to get a few good measurements of your battery to establish its behavior. Use a smart charger to get to full charge, rest 24 hours, and measure.
This is why people on boats sometimes have 2 banks of house batteries. One will rest while the other is used. When it's rested for 24 hours, they measure sate-of-charge, and then decide how to recharge - solar, wind, generator, etc. Then they charge it up, put it in to service, and disconnect the other bank to rest for 24 hours.
You can also measure the specific gravity, using a hydrometer. You don't have to rest the battery first, but you do have to correct for temperature.
Because measuring state-of-charge is so hard, people often choose to measure how much energy they use. Basically you install a carefully calibrated, low-resistance shunt on the battery terminal, then measure voltage drop across the shunt over time. If you start measuring when the battery is full, you can get a sense of how low the battery is after a while. This is typically reported in "
Ah". There are still issues: cold batteries don't give as much energy; higher currents (over a short time) cause batteries to drain faster (than lower currents over a long time); you have to be fully charged at the start to know anything useful; as the battery ages its total capacity drops.