Look at the seafood case. If both cooked seafood, such as shrimp, crabs or smoked fish, and raw seafood are sold, they should be displayed in separate cases.
Look at the packages. They should not be leaking. Juices from raw foods can carry bacteria that make you ill and can contaminate food which has already been cooked (and may not be thoroughly cooked again). Microwave reheating is insufficient to kill many bacteria.
Don’t buy frozen seafood if the packages are open, torn or crushed on the edges.
Avoid frozen seafood packages that are above the frost line in the store’s freezer.
If you are buying fresh shellfish, it should be alive. Live shellfish will show some movement like curling their tails, moving their legs and closing their shells. Discard any that die during storage or if their shells crack or break.