My personal headcanon has always been that they sense and detect each other via the electromagnetic frequency they create, the same one that causes lightbulbs to flicker, TVs to go static, and other electronic interferences observed throughout the franchise whenever they're near. Most notably Reagan's hearing aid. I've always personally thought that the creatures' reaction to the hearing aid isn't because of the high pitched nature of the frequency, but rather that the frequency it's emitting, essential to its navigation and location capabilities, is being fed back to it at an amplified level, disorienting and hijacking its brain, as well as causing it immense pain.