Poke a fork into the thickest
portion of the fish.
When fish is done, it becomes opaque and flakes. To test for doneness, poke the tines of a fork into the thickest portion of the fish at a 45-degree angle. Then gently twist the fork and pull up some of the fish.
Undercooked fish resists flaking and is translucent. If your fish is undercooked, just continue cooking it until it is done. But remember, fish cooks fast, so be careful not to overcook it.