Many buyers look for eye clarity as an indicator of freshness. Deeper caught fish such as grouper will have cloudier eyes. Bloodiness is also a good indicator of freshness. A fish with fresh blood in the cavity or veins is going to be fresh. Fish with dried blood or brown blood stains will lack freshness.
Gill color is also an excellent indication of freshness.
Because the gills are responsible for oxygen exchange with the blood they are generally deep red in color. Similar to the bloodiness characteristic, gills that are brown in color suggest a lack of freshness. Additionally, gills should be clean and clear of any mucous. But every fish species tends to be a little different when it comes to freshness. For example, halibut emits a translucent green slime up to 72 hours after it has been caught.