A lot of automobile tailights are composed of multiple LEDs.
You can usually spot them by moving your eyes rapidly past them. LEDs will leave static afterimages.
Incandescent lamps will leave a smeary trail.
They are bright enough now to do the job, and they last longer and don’t damage the plastics by overheating.
The telltale I’ve most noticed is the “instant off” property of the LEDs. Incandescent bulbs fade out. Quickly, but not instantly. The difference is noticeable.