How to choose oysters
Be sure to get your oysters (and all seafood) from a reliable fishmonger. If you don't have one, check your local supermarket -- many larger ones have excellent fish departments. Occasionally, busy seafood restaurants may also have an adjoining market open to the public.
When choosing oysters, rely on your senses:
Smell: Fresh oysters should smell like sea water; crisp and briny and not at all fishy. Pre-shucked oysters should have no ammonia smell.
Sight: Oysters should be kept on ice, in a well-drained refrigerated case. The shells should be mostly closed and should close tightly when tapped with a finger.
Touch: The oyster shells should be scratchy and may have barnacles; avoid oysters with lots of algae, seaweed or mossy patches, as these may have been kept in a tank with poor water circulation.
Taste: If you're lucky enough to be able to taste-test the oysters, check for levels of brininess and adjust your recipe accordingly.
In the United States, fish sellers are required by law to keep the delivery tags from shipments of fresh seafood. Ask to see the tags; they give the dates of shipment and delivery, so you'll know how long the oysters were in transit, how long they have been on display, and their source.