You need a digital voltmeter.
12.6 volts across the terminals is a fully charged battery not connected to a charger.
13.2 (ish) volts should be what you get under a float or maintainer charge.
14+ volts is the battery under full charge with a 3-stage converter.
12.0 volts or less is a depleted battery. Or if it's connected to shore power the converter isn't working.
If you don't have a voltmeter the battery monitor should have 4 lights on. This isn't a good indicator though.