All-when alive

Legs move when touched; rocklobster tail curls under when the animal is picked up

Little movement; rocklobster tail hangs limp when the animal is picked up

More vigorous movement signifies better quality. Chilled seafood will move less and be slightly limp.
Dead, uncooked (raw) crabs should not be purchased as it is very difficult to assess how long ago they died.
If the tail of a rocklobster or freshwater crayfish does not curl under the head when cooked, it could be undercooked or not "fresh" when cooked.

Shell condition

Not cracked, no missing body parts

Some cracks, and releases large amounts of liquid when shaken (especially when cooked); some missing body parts

If cracked, the flavour and texture of the flesh can be damaged by water during cooking. If the flesh is dry, the animal has probably "bled" or been overcooked.

Colour

No discolouration; most are bright red or orange when cooked

Black discolouration beginning to form at the mouth, head or base of tail

Discolouration often occurs first at the joints.

Flesh colour

Translucent when raw; opaque when cooked; no brown staining

Green, yellow, brown and/or with dark spots when raw; creamy, yellow or grey when cooked; slight brown staining at top of tail (not in crabs)

Living environment can affect flesh colour, and the flesh of some species has a yellow tinge. Crab claw meat tends to be darker, and is often not used where presentation is important.
Chop off brown-stained area at top of tail if necessary.

Pleasant smells, virtually odourless

Beginning to smell like seaweed or garlic when uncooked; slight ammonia smell

Smell is a reliable guide to quality. Meat in the carapace will go "off" first because it has closest contact with the guts. For crabs, lift up the abdominal flap to check the smell.
Any mud or other foreign matter should be removed before checking odour.

Shell

Hard (unless a soft shell crab product)

Soft (unless a soft shell crab product)

Check for softness by firmly pressing the underside of the shell near the base of the claw. (Note: if the animal has moulted recently, the shell will be softer.)
Mud crabs are naturally harder than Blue Swimmer and Spanner Crabs.
Weight can be an indication of value for money, but is not necessarily a guide to quality. A light weight usually means that the animal has moulted recently and lacks meat content. Crab shells should sound heavy and full when lightly tapped.

Flesh

Moist, firm and juicy

Some dryness; slightly chewy or tough

If the crustacean is stressed the meat may toughen. Undercooking leads to mushiness, overcooking usually results in tough flesh. If the animal has moulted recently, the flesh tends to be softer and not as stringy.