One way to test the charge of the battery is with a hydrometer. The other is with a voltmeter. A hydrometer shows the specific gravity of the electrolyte inside the battery. The advantage to a hydrometer is it allows you to determine if one of the cells is weaker than the other cells. With a voltmeter, a battery can show that it has enough voltage, but one cell may be weak and not able to hold a charge. In this case, the battery will leave the shop with a full charge but let you down later in the day. A graduated float-style hydrometer will give the most accurate readings.