How do I know the shellfish I purchased was safely harvested, processed, and merchandised?

The FDA requires shellfish harvesters to put a tag on sacks or containers of live shellfish and a label on containers or packages of shucked shellfish.  Tags and labels contain specific information about the shellfish, including a certification number for  the processor.  It  indicates that the shellfish were harvested and processed in accordance with national shellfish safety controls.Ask to see the tag or check the label when purchasing shellfish. 
 
Also, be sure shucked shellfish is properly refrigerated or iced.
 
Live clams, mussels, and oysters should have closed shells. If they are slightly open, they Should close tightly when lightly tapped. Live crabs and lobsters will show some leg movement and lobsters will curl their tail when handled.
 
Source(s):
FDA "Fresh and Frozen Seafood: Selecting and Serving it Safely"
Nova Scotia Agriculture--Food Protection and Enforcement Division "Shellfish Food Safety"