A battery hydrometer tells you acid concentration, which indicates state of charge. (Assuming the battery has had time to 'settle'.)
A voltmeter tells you its state of charge, although it is really better for alerting you when the battery is close to discharged, or is overcharged. A charge controller measures the rising voltage as the battery charges, and stops when it reaches full, which is typically 14.4 V.
The best indicator, if it is possible to monitor each charging session, is to use an ampere-hour meter.