Only practical way to test its capacity is to fully charge it (should read 12.7 volts or more after resting overnight without load), then put a known load on it and measure voltage decay over time.
Most marine batteries
are rated for 20-hours, so it's best to put a load on it which, theoretically, would pull the voltage down to 10.5 or so (fully depleted) in 20 hours. Example: if you have a gelled battery rated at 20AH, put a 1A load on it (about 12 watts) and let it go for awhile. You don't have to go all the way to 10.5V to get a good idea of its capacity.
Another way, not terrific for deep cycle batteries but not bad either, is to borrow one of those expensive testers which measure internal resistance and give you a readout in CA, CCA.