Right, but this is what I meant to ask: how does one verify a switch that has been manufactured by someone else? I suppose that an electrical engineer could check the schematics, but how do you verify that the actual hardware in your device matches the schematics?
Sarcasm, I hope? I spent an hour this weekend replacing some micro switches in some decrepit robots. These are about 5mm^3, which is pretty tiny, but huge on the scale of a modern phone. They are basically hunks of plastic with a button on one side and six or so leads coming out the other side. No, you can't open them up and see the air gap without destroying them. And yes, they are small enough to easily contain a full CPU, some flash memory, and a few sensors, even though they only really need a few sliding metal and plastic bits.
Physics. Their not being an unnoticed second camera/microphones/etc.
If you main concern is that your computer will be used to spy on your physical life, then having verified switches can give you confidence from first principles that it it has no way of observing you (modulo secondary sensors that you did not notice.)