External alternator Lamp

The lamp only illuminates when the Generator output is very low.

It is about as simple of an indication as you can get.  (1920s Technology)
One side of the bulb connects to the battery.
The other side connects to the Generator output.
If the Generator is near zero volts, there is near 12v across the lamp. It illuminates.
If the Generator is making 12 volts, there will be 12v on both sides of the Lamp; Zero ACROSS the lamp. It will be Dark.
The whole circuit is a Lamp and a diode. It is about as simple as you can make it.
It may very well be the first Idiot Light invented.