What does the ambient light sensor look like?
The sensor is like a small dot, usually located on the left of the HTC logo, above your device's screen. Also the location of the proximity sensor is.
FYI: HTC ONE models S, X, V, EVO 4G LTE are very much alike in physical conception.
This is covered at XDA Developers forum thread: Where is the One X's ambient light sensor?

By Tiersten:
Left of the HTC logo above the screen. It is also where the proximity sensor is.
By jaaystott:
If you call someone, take a look at the front of your phone. You will see a steady red glow from the top left of your screen, this is where the proximity and ambient light sensor are housed.


Additionally, there's this White Paper: Ambient Light Sensor (ALS) Applications
in Portable Electronics that shows their physical aspect: