The hydrometer will show you the specific gravity of the electrolyte in the cells - and then you would need a digital voltmeter to check your voltage
SG should be 1.250 to 1.270 for proper SG - higher - you have doped it with acid - low means its either down or very dilute
the standing voltage check is done with car rested for two hours then, the headlamps turned on for 2 minutes - switch off - then read the battery volts
that should be 12.6 for a fully charged battery, anything less than that is not fully charged and a battery with a reading low as 11.9V means its completely dead.
difference of .50 points on the SG scale between any 2 cells means the cell is dead - replace battery
Proper SG with low voltage = sulphated battery - throw it away
higher SG with high voltage = too much electrolyte - add water
lower SG with low voltage = too much water - add 1.400 SG acid