A voltmeter is any device that measures the difference in electrical potential between two points. You'll typically use a device called a multimeter to measure voltage; a multimeter also measures other electrical quantities. A common application of a voltmeter is to measure the actual voltage of a 12-volt battery.
Locate the settings on the multimeter's selector knob for direct voltage. This group of settings should have a label such as "V" or "V=." Each setting in this group should be labeled with the maximum voltage that the setting can measure.
Set the selector knob for the multimeter. In the case of a 12-volt battery, you already know that the voltage won't be much higher than 13 volts. You may therefore set the selector knob to the lowest setting with a maximum voltage higher than 12 volts. This will frequently be a setting labeled "20V."
Insert the plug for the black lead into the jack labeled "COM," "-" or something similar. Insert the plug for the red lead into the jack with a label that contains the characters "V" or "+."
Connect the alligator clip for the multimeter's black probe to the battery's negative terminal. Connect the alligator clip for the multimeter's red probe to the battery's positive terminal.
Interpret the voltage of the 12-volt battery. A fully charged 12-volt battery should have a voltage of 13.2 volts that will slowly discharge down to 12.65 volts. A battery with a charge of 75 percent should have a voltage 12.45 volts, and a battery with no charge at all should still have a voltage 11.89 volts.