Most modern vehicles have an indicator light located on the dash to alert the driver of a malfunction in the charging system. It is also used to pass a small amount of battery current to the alternator rotor to excite and produce the magnetic field until the alternator begins to charge and can assume this function itself.
Because the bulb circuit is connected to the battery and alternator sides, any movement of current between the two units will cause the bulb to light. As the alternator begins to charge and the produced voltage reaches the battery voltage, the current between the two units ceases to move and the bulb will go out. If either the battery or the alternator should fail as the vehicle is being driven, the difference of voltage between the two units will allow current to flow and the bulb to light, warning the driver of a malfunction.
An ammeter also indicates the condition of the charging system. A low battery will be indicated by a high charging current toward (+) side of the gauge. A wiring short or faulty accessory will show as a high rate of discharge toward (-) side of the gauge. It's normal for the gauge to move a slight amount in either direction.