 Are they collapsed? Is the skin still shiny? Are the scales intact? Are the gills moist with a bright red color? If the fish you're staring at fails to check any of those boxes, you better look for another one. But if your answer is yes to all of those, again, that's the key. Are the eyes clear? They haven't collapsed? Is the skin still shiny? Are the scales intact? Are the gills moist with a bright red color?