One of the hardest times to stick to a low-fat diet is when confronted with a restaurant menu. What's really in all those dishes described so temptingly? Since the average American eats several meals a week away from home, it's important to know. A new pocket-size book can provide some guidance. The ``Guide to Low-Cholesterol Dining Out'' by Harriet Roth (Signet, $1.95), focuses not only on the animal products that contain cholesterol but on high-fat foods that can raise the level of cholesterol in the body. Roth advises diners to ask questions and make requests (such as sauce on the side). More and more restaurants are willing to comply with health-conscious eaters' wishes, said Roth, former director of the Pritikin Longevity Center Cooking School. There are general guidelines and lists of foods to enjoy and avoid at 17 kinds of restaurants, in take-out foods, among beverages and when traveling. She also includes a brief list of requests (such as ``I am on a special low-fat, low-cholesterol diet.'') in French, Spanish, Italian and German. ---