You will notice the fish change color, it "whitens" a little. If it has a bone in it you will notice that it tends to pull away from it. The fish should flake easily with a fork once it is done. Do not cook longer or it will be tough.
This does depend on the cooking method but if you are baking it in the oven, about fifteen minutes should be sufficient, baking at 350 degrees. You will know it is done when it flakes easily with a fork.