Oh, dear, I might be wrong but I've always tried to be careful when buying fish, especially since I'm trying to eat healthier these days. I mean, nobody wants to get sick from bad food, right? So, what I usually do is look at the eyes first. I think they should be clear and shiny, not cloudy or sunken in. It's like they're looking back at you, almost. Then, if it's a whole fish, maybe try to lift the gill flap? I believe they should be a nice bright red color, not brown or slimy. I don't know for sure, but I've heard that's a good sign. Also, maybe try to gently press the flesh with your finger. If it bounces back quickly, that's supposed to be good. If it leaves a dent, it might not be as fresh. And, of course, the smell. It shouldn't smell too "fishy," if you know what I mean. More like a fresh, clean ocean smell. I hope this helps a little, it's just what I've learned over the years.