Shell

Clean, intact

Chipped or dirty

Mussels have byssal threads (filaments) that are used to attach the mussel to a surface. These may be present but do not affect the eating quality.
In live animals, gaping shells suggest that the animal is dead or dying and should be discarded (it may have been in this condition for some time and could be contaminated).

Liquid (liquor)

Clear

Slightly opalescent (oysters), or slightly milky in colour

Flesh Texture

Juicy, moist, plump and firm

Starting to shrink and look flat

Oysters are often turned over when shucked to give a "plump" look.

Colour

Mussels are either white or orange; oysters vary naturally from greenish grey to white or ivory

Mussels fade; oysters darken (but may be a brilliant white or greenish)

Living environment and feed can influence flesh colour.
Mussels range from cream to pale or deep orange with a black rim. Female mussels are orange in colour; males are whitish.
To determine if an oyster shell is original or re-used, check for a muscle remnant. In a "freshly" opened oyster, presented on a half shell, the muscle that attaches the flesh to the shell will have been cut, and the flesh turned over. When the shell is re-used, it is cleaned and the muscle remnant removed. Note re-using shells can be a health risk, and may be in contravention of food laws.