Greg Mayman wrote in a message to George White:

 GW> rpm. On my car (less than 10 years old) the alternator goes off
 GW> regulation with the headlights on at somewhere around the 1000 rpm
 GW> mark, idle is at 700 rpm... You can see the headlights dim as the
 GW> alternator goes off regulation.

 GM> Which is a useful indication that the alternator is putting out
 GM> current -- on most cars, about the only indication.

I'm used to vehicles with ammeters...   Most of the old mopar stuff I've
been driving for years has had them in there.  Except this last one,  which
for some dumb reason has a voltmeter,  instead.  Why they started doing
that is beyond me.

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