Look at the empty brood cells not the cells filled with pollen or honey. There should be a large section of dark comb that is empty. Frass will be on the sides and easy to see. If there is a capped brood section look for pin holes in the cap. The bees poke a hole in the cap to kill the brood and that kills the mites.
LJ, that doesn't look like brood come to me. Those specks could be pollen.