How can I tell if fish is fresh?
As a rule of thumb for a whole fish, and not a fillet -


the gills should be bright red
the skin/scales should be bright and shiny like metal
this fish shouldn't really smell of anything except 'watery'
the flesh should rebound quickly when pressed
the eyes should be bright and clear
really fresh fish is also quite slimey to touch if it's straight out of the water. I remember being quite surprised at this from my first fishing trip a few years back.