Believe it or not, the best way to tell if fish is done perfectly is to take a peek. As fish cooks, its flesh loses its shiny, almost translucent quality and gradually becomes opaque. To take a look, poke the tip of a paring knife between two flakes. Not only will you be able to see if the flesh is opaque throughout, but you’ll also be able to feel that the flesh has gone from flimsy to firm. It shouldn’t, however, flake apart—that’s overdone.