Buy the Fish at the Shore
Often times, fishermen drop off their fresh catches nearby the shore. You have a higher probability of having fresh fish. Try to get fish that was caught within the last day or two.
The Eyes Should Be Sticking Out
Take a look at the eyes of the fish. If they are sticking out and are shiny, then the fish should be fresh. If the eyes are sunken and darkened then the fish might be overdue already.
Smell the Fish
The fish should have a smell of the sea and of seaweed. If the fish smells fishy, then it has probably been out of the sea for too long.
Rub the Scales
The scales should be shiny and should be adherant to the flesh of the fish. If the scales peel off too easily by rubbing the fish, then it might be old already.
Check the Gills
The gills should be separated, not stuck together. The color should be red. Brown indicates that the fish has been sitting for a while.
Feel the Flesh
The flesh should be firm. If you touch it and you leave marks with your fingers, then the fish is no longer fresh.