Testing a car battery with a multimeter is a great idea to ensure it's functioning properly. First, set your multimeter to 20V DC. Then, connect the red probe to the battery's positive terminal and the black probe to the negative terminal. A healthy battery should read around 12.6 volts. If it’s under 12.4 volts, it might need charging or replacing. Have you experienced any issues that made you want to test your battery?