If the fish is whole, there are several things to look for: the eyes should be clear and bright, the scales should be shiny, the gills should be a healthy-looking pink or red and the flesh should feel firm and resilient to the touch. Fillets and steaks should also be firm and moist, and the flesh should have a fresh, translucent color.
A fresh, clean smell is another good sign. Fresh fish should not in fact smell 'fishy', but saltwater fish should smell of the sea.