The thing is, you're always going to be at risk. Even the best mongers will occasionally get a sick fish (although, they will typically know what to look for). The best thing to do is just learn what to look for yourself.

But having a trustworthy monger will go a long way in helping you with that. As Alton Brown and Emeril always say, "It pays to get to know your vendors." Not only do you get a sense of whether you can trust them or not, but if they like you, they will go out of their way to help you. Become friends with the people you buy your fish from.

Bottom line: take it from the guys who feed people. Know what to look out for in whatever product you use.