Use a multi-meter (voltage meter) and trun on the engine.
If you donít have good access to your alternator, then you can simply test the alternator by touching the positive multi-meter probe to the positive terminal on your battery and the negative probe to the negative terminal on your battery.
A car alternator needs to be putting out between 13.8 and 14.2 volts to effectively charge a 12 volt car battery. If it is putting out too much voltage (over 15 volts), battery acid will leak out of the battery. When a battery is near fully charged the alternator will stop putting out sufficient voltage to charge the battery and the meter will read only the voltage coming from the battery itself. In this case, you can leave your lights on with the engine off for ten minutes to drain your battery a little.
It can also be helpful to test at the battery terminals with the engine off and then turn the car on and test again at the terminals. If your battery is more or less fully charged, it should read at around 12-13 volts with the car off. If youíve run your car for a long time and while the car is running the voltage reading is in the 13-14 volt range, but then you shut the car off and the battery voltage instantly drops to well below 12 volts (10 volts or under; 9 volts or under in freezing weather conditions), it is likely that your battery needs maintenance or replaced.