The often-quoted theory of cooking fish for 10 minutes per inch of thickness may be a good guideline, but in reality 8 minutes is a better timeframe in which to at least start checking for doneness.
Fish will continue to cook for a minute or two off the heat. Be sure to stop cooking when the fish is just shy of done; otherwise, it will overcook by the time you serve it.
Use the tip of a small knife to peek at the interior of the fish. Many cookbooks tell you to cook fish until it flakes; this is too long. Once it flakes, the fish has lost too much moisture and will be dry and bland. As you peek, see how easily the fish gives way. It should gently resist flaking but show signs of firming. If the fish is on the bone, the flesh should lightly resist pulling away from the bone.
Raw fish has a translucent appearance that turns opaque during cooking. Most types of fish are considered done when they're just opaque throughout. Many people, however, enjoy some types of fatty fish, such as tuna and salmon, a little less done. These should be opaque on the outside but still translucent at the center.