Secondly, you want to buy fish that has been handled properly. But traveling to Alaska or the North Atlantic is not the most practical for an evening meal. Odor is the only universal characteristic that can be used across all fish species. That depends on the species. For example, halibut will have a fresh oaky smell and whitefish smells like cucumbers.
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.