You can check the battery voltage directly while the car is off to tell whether it is ok (green LED on) or low (yellow LED on). Actual testing begins when you start the car. The starter draws a lot of current while cranking, so the yellow LED should come on indicating that voltage is low but still good. If the red LED comes on while cranking and/or does not start at all, this indicates that battery may need recharging or the battery needs to be replaced if the red LED still comes on after recharging.
Once the engine starts, the green LED should start blinking for half a second on and half a second off. This indicates that the alternator is charging the battery. While the engine runs idle, you can turn head lights on, A/C on, radio on, windshield wipers on, etc. and the green LED should continue blinking indicating that alternator is charging the battery properly.
Run the engine at about 2000 rpm with the above mentioned loads on and the green LED should continue blinking. In the event that the red LED starts blinking, this indicates that the battery is being overcharged which could damage the battery and corrective action should be taken.
Only one LED comes on at any given time which makes it easy to interpret the results.