In order to measure the condition of a battery, normally two parameters are taken, specific gravity and voltage. The specific gravity is measured using a hydrometer while the latter is obviously measured using a voltage meter.
The hydrometer is an arrangement in which a float is placed in a cylindrical glass tube. The glass tube has a rubber bulb at one end and a rubber tube attached at the other (see diagram below). A scale is drawn on the glass tube, against which the level of float is measured. In order to measure the charge, electrolyte from each cell is taken in the glass tube and the specific gravity is measured. It is necessary that all the cells have almost the same charge. This specific gravity reading is related to the charge of the battery and must be corrected for the temperature of the electrolyte. For example, the approximate value of a fully charged lead-acid battery is 1.280 at 15 degree Celsius.
However do note that the value of this specific gravity at the other end of the spectrum, namely fully discharged battery is just around 1.120 approximately, which shows the narrow band width of the same.
Apart from checking the specific gravity, you also need to the check the voltage. Normally the battery would show a voltage which is slightly above its rated voltage. E.g. a battery of 12 V should show nearly 12.6 approx in order to indicate that it is fully charged. A value near to the actual rated value or slightly less (say 11.9 V) certainly indicates a discharged condition.
In actual practice, these values are not taken in isolation but both specific gravity and voltage are checked and compared with a standard chart for comparison provided by the manufacturer