Dan D.
Desperately Seeking Sapience
It can work by looking at whether other vehicles have their wipers on. Or other visual cues, after all humans can see it's raining without any windows at all.
Other good cues are the sound of rain on the roof, and yes leverage the other cameras to determine whether it is raining. (somewhat /s I suppose).
I think the point of the autowipers though is to keep the windshield clear, not to determine whether it's raining. You don't need wipers when you're sitting under the overpass in a traffic jam. You do need wipers when someone splashes your windshield by driving through a car wash puddle on a dry day.
I'm not sure if auto wipers will ever work. If it is a light rain/drizzle it is often that the upper part of the windshield doesn't get any rain due to it being blown away by the wind so the camera does not see any raindrops while the lower part is getting wetter and wetter until I can't see anything.
Still, wouldn't cars with a rain sensor have the same problem, if the rain drops are being "blown away by the wind"? They are often mounted up high by the rear view mirror just like Tesla's cameras.