Uncooked crabs should be alive as indicated by leg movement. Movement of refrigerated crabs may be slight.
Scallops turn milky white and opaque in the center when cooked.
The flesh of shrimp and spiny lobster should be firm. Shrimp become tender and turn pink with an opaque or white, rather that translucent center. Generally medium or large shrimp will cook in only 2 to 3 minutes in boiling water. Oysters, clams and mussels in the shell should be alive, as indicated by a tightly closed shell. Fresh shucked oysters should be in a clear slightly milky or light gray liquid, called liquor.