Yet another test is to cut a small slit in the top with the tip of a paring knife. Remember that fish will continue to cook even after you've taken it off the grill. As with so much in cooking, your final results will be only as good as the raw materials. Fish should smell fresh and clean, not fishy. Whole fish should have bright gills and clear, glassy eyes.
Many fish benefit from a light smoke flavor.
Soak some oak or hickory chips in a pan of cold water for 1 hour. If using a charcoal grill, toss the chips on the coals just prior to putting on the fish. If using a gas grill, wrap the chips in foil, make a few holes in the top to let the smoke out, and place the foil bag under the grate over one of the burners. When cooking whole fish, make a few diagonal slices in the side of each fish to the bone. This helps the fish cook more quickly and evenly.