Your brake rotors will generally give you signs when they’re starting to wear too thin. The forming of a lip on the edge of the rotor is a sign, as is any glazing on the finish of the rotor. You may also feel a spongey response form your brake pedal, and vibrations under braking while driving. You can machine your rotors to restore the finish, but as this can bring them to within unsafe thickness levels (and cost a lot of time) we’d generally recommend replacing them. For more detailed information, check out this article.