If in doubt, chuck it. Food poisoning is not pleasant. But don't just use your nose, look at the texture of the flesh. If it doesn't look like it should, chuck it. If you really don't want to waste it, try cutting a sliver of the fish and eating it. If it tastes alright, well you are in with a shot. However, I would try a very simple dish if you go ahead and cook it (like pan frying in herbed flour), one that lets the flavours come thru, that way if it is on the turn, you will know very quickly. If you oven bake it or smother it with a sauce or marinade, you could end up unwittingly serving bad fish to your diners.