You can check battery voltage with the car running and all the lights on, radio on, etc, etc. Note the voltage. Then, turn off all the lights and radio, etc, etc. Note the voltage. With a strong battery/alternator, the variation should not be much. It is a very basic test but, it will give you a rough idea where you are. Another thing you can do is take the regulator with brushes (they come as a unit) out of the back of the alternator (2 screws) and check the length of the brushes. When they are new, they are about an inch long. If they are short or close to the base, replace them.