Flesh colour

White, slightly translucent when raw

Darkens to a strong pink tinge

When "freshly" caught, squid have an attractive translucent flesh. If you put your fingers inside the tube, you should be able to see them clearly. After death, chemicals are released that discolour the flesh.
This process can be aggravated by damage to the internal organs through rough handling. Discolouration is clearly visible when the skin is removed.

Skin condition

Intact, with distinct patches of iridescence in squid and cuttlefish

Some tears in the skin, some broken arms

If trawled, the skin may have been rubbed off or torn; this does not necessarily indicate poor-quality flesh.
Ink on a squid or cuttlefish does not mean poor eating quality, but is a warning of poor handling. It can encourage bacterial growth and should be washed off immediately.

Flesh

Elastic, very firm, not slimy

Loss of elasticity, soft, slimy or gritty

Squid and cuttlefish are often perceived as being overly tough. The flesh is naturally firm but can be toughened by incorrect cooking or overcooking.
Grittiness can be caused by foreign matter such as sand.
Octopus is naturally firmer than squid and cuttlefish.