How do you know if alternator is bad or the belt?

Make the sure belt has tension in it and it's turning the alternator pulley. When the car is started, remove one of the positive battery terminals (carefully of course and don't let it touch any other metal, wrap it in a rag). If the car continues to run, then the alternator and volt regulator are doing their job. The alternator's primary function is to recharge the battery after starting the car, then to run the vehicle's entire electrical system, which includes the ignition system. If the car dies after you remove the battery cable, then most likely the alternator is working. The car may not remain running when a battery terminal is removed. An alternator uses coils instead of permanent magnets for their stator. In order for their to be a magnetic field in the stator, it requires some current to be passed through the coils (A generator uses permanent magnets, so it already has a magnetic field in it without needing any external current). This current comes from the battery and if the battery is removed, the alternator will not put out any current, ultimately causing the car to stall out. However, if the engine uses a generator, it would remain running. Consequently, an alternator will not charge a completely dead battery.