For fillets or steaks, use the finger- poking method. When pressed with your index finger, the fish should spring back. If the flesh is mushy or leaves a permanent indentation, it's not yet done. If it flakes, it's overdone. Whole fish are a little trickier, though the finger poking method still works.
Fish is done when it's opaque rather than translucent.
Shellfish, with their delicate meat, require even more care on the grill. Shrimp are a natural, the larger the better. Ditto for large scallops and soft shell crabs. A few minutes on either side is all you'll need for any of them, unless the shrimp are unusually large. Mollusks such as oysters, clams and mussels are fun on the grill.