All condition

Head firmly attached, not squashed

Head loose; body may be soft and slightly broken

Soft and broken shells may not be an indicator of poor eating quality; the shell may be soft because the prawn has just moulted.
The tail being tightly curled underneath and a little separation between the flesh and shell mean that the prawn is cooked; they are not indications of any health risk.
Intact antennae can indicate that the prawns have not been frozen.

Colour

Bright, glossy, without signs of fading

Beginning to darken around the edges of the body segments, legs, shell, flesh, gut or head areas; some fading (particularly with certain species); dry, bleached areas

Black spot or brown head need not mean that the flesh quality has been affected.
Yellowing around legs can be caused by excessive use of metabisulphite, which is used to control black spot.
A dark intestinal tract can indicate roe.

Flesh texture

Firm, but not exposed if shell-on

Soft, slimy or gritty (if raw); chewy, soft or watery (if cooked)

Grittiness can be a result of undissolved metabisulphite. Overuse of metabisulphite can be a safety issue.
Large prawns are not always tougher than small prawns. Tough texture can be caused by overcooking or being held in brine too long. Prawns can become soft and watery after freezing and thawing.

Sweet and distinctive prawn flavour

Salty, or reduction of distinctive prawn flavour

The saltiness of a prawn can reflect the amount of salt the prawn has been exposed to (for example in cooking or storage). Saltiness can hide a lack of flavour. Overuse of metabisulphite can result in a slight chemical flavour.