Chef Anthony Bourdain, in his acclaimed book Kitchen Confidential: Adventures in the Culinary Underbelly, lists a few things he's weary of when eating out.
Here are some of them.
He avoids 'Discount Sushi'.
He never orders fish on Mondays (in NY restaurants). According to Bourdain, the fish market in New York is closed on Friday, therefore fish is ordered Thursday for the busy weekend and the second order goes in Monday for Tuesday. As such, the fish served Monday is 4-5 days old. Tuesdays and Thursdays are the best days to order fish.
Tuesdays are the best days to eat out. Chefs usually get Sunday and/or Monday off and food orders come in for Tuesday... so the chef is well rested, the food is fresh and it's not busy, allowing the chef and staff to take care of you. Food is fresh Friday and Saturday but the kitchen staff is really busy therefore aren't paying as much attention.
Bourdain doesn't eat mussels in restaurants unless he knows the chef or seen for himself how they're stored or cooked. In only takes one bad mussel to make you very very sick.
He avoids Hollondaise sauce. This sauce is an emulsion of egg yolks and clarified butter and has to be held at room temperature. Bourdain has never known of any restaurant to make this sauce to order, meaning it sits at room temperature for a long time, allowing bacteria to fester.
Bourdain points out that in most restaurants, the bread is recycled off of people's tables. Although this doesn't bother him much, if it bothers you, you may want to avoid it!
He doesn't eat in restaurants with dirty bathrooms. Bathrooms are relatively easy to clean, kitchens aren't.
According to Bourdain, the key is rotation. A varied menu and a half empty dining room is a bad combination.