One day Just Allan got dressed and committed to text
One method is to measure the volts under load with an accurate voltmeter such as a digital volt-ohmeter. Most manufactures say that if it drops below
10.5v then deteriation (SP?) starts to occur. Personally I stop it at 11v to be on the safe side. There should be graphs from the maker somewhere to judge the remaing capacity. Google for Bill Bowdens battery faq.
You can be sure a battery is fully charged - use a good voltage regulated charger - some of these even have two or three "program" steps - initial charge, mild current force for capacity extension, float. You can leave your battery on a good charger for as long as you like without damage. Good solar chargers also do not overcharge.
Once you know your battery is charged, you can estimate the depth of discharge - amps x hours.
I don't recommend constant use of a hydrometer, because you lose a little acid each time you take a reading, and after even a dozen readings, you may need to top up again. Unfortunately, you have lost sulphate ions, not just water, and you would need to top up with acid to replace them. All too hard and I have messed up batteries doing this.
If you go to the Yuasa etc websites, you can get datasheets for sealed types. For deep cycle, battery manufacturer websites generally offer data about state of charge vs voltage, providing current drain is small or has been discontinued for some time. Deep cycle batteries are generally not drained rapidly compared to automotive types, and voltage will be useful even under load.
A intelligent charge meter would be the hi-tech way to do it. They can track how much charge goes into the battery and how much comes out, among other things. Don't know if you can get them for car batteries though, but chips are available for LiIon battery packs etc.
I reckon you'd only get a couple of readings that way !
As part of my job I sometimes do a hydrometer reading on the same battery every few days, over a number of charge, settle, discharge cycles, recording battery characteristics. I notice that the electrolyte level gradually drops, eventually requiring top-up. If you think about it, you always lose a drop here and there, and you have to wet the hydrometer. So hydrometer readings are not much good long term - say every day when you want to check charge level on a deep cycle installation.
I have noticed that the simple act of adding distilled water to a battery changes the battery characteristics to the extent that I can't use it any longer for characteristics measurements, and the battery takes a long time to recover.
On that basis, I would not want to spoil an expensive deep cycle battery by juggling with acid - all too clever - I would rather leave the sulphate ions inside that battery to begin with.