This is my experience; I have a 54 New Yorker, there is no amp meter. I did not know that the amp warning lamp on the dash (which illuminates along with the oil light) is crucial to the charging system. That bulb had burned out thus while driving the car the battery was being drained. Stopped for gas and the car was dead, that is when I learned about this. Towed home, charged the battery, ran the car, no charge at the battery. Someone told me about the bulb, checked it and found it burned out. Replaced bulb, charges fine. Lesson learned, I always look for the amp light before starting the car.
Additionally, just as a back up; I bought a small volt meter that would be used for electronics projects (made to be mounted in a panel). I used a old cell phone charger, dissected the plug, switched the polarity on the center contact, cut the plastic housing and mounted the voltmeter to it. When plugged into the cigarette lighter socket you can get a constant digital reading of the voltage. I run an OPTIMA battery. On start up fast idle it the volt meter shows about 7.5 V. Normal cruising about 6.8, stopped in traffic about 6.0.