Restaurant chefs have an ingenious way of checking whether a piece of fish is properly cooked without having to cut it open, and it's something home cooks can easily do, too. Simply plunge a small sharp knife all the way through the thickest part of a fish fillet and hold it there for 5 seconds. Pull it out and carefully touch the flat side of the knife to your lower lip, which is very sensitive to temperature. If it feels warm, the fish is just cooked through and ready to serve. If the knife is still cold or cool, the fish needs more time.