Apple - Where is the True Tone ambient light sensor on the 2018 MacBook Pro?
I know this has been answered but I thought an image may be useful to anyone else looking for the answer.
The sensor is 1/2 inch to the right of the camera.
I pointed the display at a bright light and opened the Display system preference. I moved a small object along the top of the screen until they brightness changed.
Thanks to Solar Mike for the idea.