Disconnect the charge warning light wire from the alternator then start car, put metre on battery not reading touch a LIVE wire from battery onto the alternator charge terminal, the engine note will dip very slightly and the volts should rise.
The alternator uses the LIVE through the bulb to initiate the charging process, when it excites the field windings in the alternator and starts charging it then outputs 12v down the terminal(charge light) and so puts bulb out, as two lives don't make a bulb light.
With this method you can watch the actual difference between running with no charge and charging.
tabetha