I was on a trip a while back and my alternator went out without me noticing right away.
We were traveling during the day and not using the headlights, so the battery voltage hung in there for quite some time.
I had a fairly new battery in the truck and I was able to drive toward home close to 100 miles before the battery voltage dropped low enough that everything but the engine quit working.
The dash gauge cluster quit working and that was really my first clue that something was wrong.
Without the voltage gauge, I was not able to determine at what level of battery charge I had.
I have since added a basic digital voltage meter that plugs into the cig lighter to my travel tool kit.
The windows would not roll down and the a/c and the radio stopped working.
Finally, the battery voltage dropped to the point the engine's electronics could no longer function and the engine just died.
Luckily, we were fairly close to home and my brother in law came to the rescue.
He jumped the battery and got enough voltage in it to allow me to drive the rest of the way home.
I now carry a spare alternator in my travel tool tub, just in case.
The point of the story is to check the voltage output of your alternator.
If the alternator stops charging, your electronics will begin to drop off with the engine electronics being the last to quit.