The "tool used" (or tools) can very depending on which modality you select. Carborundum Dressing Stone, Diamond Dressing Stone, or a "spinning diamond dressing system" which is the fastest method yet perhaps the most challenging if not experienced with it.
Maybe someone else can speak better to "depth indicators?" I dress the spinning stones "by eye" until all deviation, wobble and variant in the stone is removed. As the "out of true" stone is spinning, the "truing stone" is pressed against it. Some rest it on a registration support (the most common method) while others may hold the "truing stone" free hand. I'm not as concerned with how much depth I remove, as I am with the stone spinning "true" and the surface also being return to smooth and "true."