The only tool that you will need to test your car battery is a voltmeter. Here are the steps needed to safely and accurately test the car battery.
Turn the car engine off. Make sure the lights are off. Open the hood and check the battery terminal for corrosion. Use a wire brush to make the terminal connection clean.
The red clamp on the voltmeter is positive, and should be connected to the positive end of the battery. The black clamp on the voltmeter is negative, and gets placed on the negative battery terminal.