Hook up volt meter to battery and check voltage. Have some one start the car - check the voltage. Rev the engine to about 1500 rpm check the voltage. If the voltage does not go up when you start the car - most likely bad alternator. But I have had a case where the wire from the alternator to the battery broke internally - so before you condemn the alternator - follow the wire back to the alternator and check the voltage at the wire there with the car running. If it still is not 13.32 volts or higher - the alternator is bad.