We understand your concern how to differentiate fresh seafood from not so fresh. Here is a guide which will help you buy fresh food each time.

Go for the bright and clear eyed fish. Dull eye is a way of fish telling you that it’s not fresh at all.
Yes, smell the fish and if it smells like seawater. Never ever buy a bad smelling fish. Also, if it smells like strong chlorine, drop it. Some fishermen coat their fishes in chlorine to sell their old catch.
Fresh fish exhibits red rich color of gills. If not fresh, gills will be like of faded brick color.
Perform the same drill here as you did while buying fish.
Eyes must have a protruding appearance in the head and should appear shiny or glossy.
This should also smell like seawater. Never ever buy a bad smelling or chlorine smelling prawn.
Drop dry and tough prawns and buy moist and firm.
It should have a shiny and good appearance and if there is a black spot on the shell, it’s no longer fresh.