Before Testing: Important Safety Precautions
A car battery packs a lot of juice, so safety is extremely important. Before even opening the hood, turn on the ignition to make sure your car starts as normal. Then, with the engine still running, make sure your headlights and any sidelights are switched off. Also, turn off any other accessories, such as the radio, air conditioning, GPS and so on.
Be sure to switch off any devices that can still run with the engine off: you want no electricity flowing through the system at all. Once you've done all this, turn off the ignition again.
Finally, since lead acid batteries contain dangerously corrosive contents, it's a sensible idea to wear eye goggles and sturdy, waterproof gloves to protect you if the battery has a leak or other problem.