I don't think they have sensors - everytime I get mine serviced, the guy says they are getting low, but fine for another 10k so he'll check 'em next service. Been saying that since 50k miles.
Thing is, most of my mileage is motorway so I am not actually braking that much. I think I must be quite light on my brakes as I had put over 60k miles on the fronts on my last car before I got rid of it & there was still plenty left.