A forklift battery is a lead acid battery that is constructed much like automotive and truck batteries. It uses sulfuric acid to activate the cells of the battery and provide electrical current. The sulfuric acid can be tested using a standard battery acid tester which is also known as a "hydrometer." This test will give you the specific gravity of the acid inside of the battery, which will allow you to calculate how much voltage you have left. Since there are many variations and voltages of forklift batteries, hydrometer readings may vary.
Open the battery storage compartment on the forklift. Remove a cap from one of the forklift battery cells.
Insert the acid testing hydrometer into the cell and push in the bulb at the top. Slowly release pressure on the bulb; battery acid will be drawn into the hydrometer.
Pull the battery acid tester out of the cell and read the specific gravity. Markings will be located on the side of the hydrometer. A common specific gravity reading will be 1.265 for a fully charged cell, and 1.13 or less for a completely discharged cell. These figures may vary from battery to battery, but they are a good base line to work with.