It is easier than you think
Fresh fish has a distinctive odour rather than an objectional smell. Characteristically, fresh fish have clear bright eyes, red gills and tight, shiny scales covered with a natural slime. The flesh should feel firm and not separate from the bone easily.
Live lobster and crab are rigid and lively rather than looking limp.
Live oysters have tightly closed shells. When two oysters are tapped together, they will produce a sharp crack, not a hollow sound. Live clams also have tightly closed shells. The shell of a mussel might be open, but if the mussel is still alive, its shell will close tightly when pressed together.
When buying whole fish, allow one pound per serving. For dressed fish (entrails, head and tail removed) or steaks, allow one half pound per serving. Allow one pound of fish fillets for every three adults.