IF ANY idiosyncrasy separates American cuisine from all others it is the salad bar. Salad bars soothe the conI science of the sporadic dieter, while offering an alternative to fattening entre'es for those who often eat out. Salad bars fit any life style -- they are as welcome among burgers as among steaks, allowing vitamins to creep into a diet where the only other vegetable is likely to be french fries.

But the salad bar should be approached with caution. While it consistently offers a source of fiber, minerals and vitamins so often absent from the American diet, its cornucopia can be abused. According to Madonna Heckman, registered dietician at Providence Hospital in Northeast Washington, the opportunity for an extremely nutritious light dinner can disappear quickly under a mound of croutons, bacon bits, grated cheese and thousand island dressing.

Of the people who line up at the salad bar, says Heckman, most "think they're going to get a well-balanced, nutritious meal." What they don't think about are the hidden fat, sugar and sodium that may lurk in the selections.

Very few salad bars, for instance, offer low-calorie salad dressing. Consequently, it's the dressing that usually adds many times the calories of the unadorned salad. Most often, this culprit is based on oils (which usually run 125 calories per tablespoon), mayonnaise (110 calories) or sour cream (26 calories).

But calories sneak in other places, says Heckman. Garbanzo beans, nutritious as they are, contain about 166 calories per cup. While you probably won't dump a cup of beans on your salad, these and other legumes (such as kidney and navy beans) can add unwanted calories.

Cheese is also calorie-dense. Swiss and cheddar cheese contain about 100 calories an ounce, or 30 calories for a tablespoon of grated cheese. A tablespoon of bacon bits contains about 50 calories.

Obviously calories can add up quickly. Thus, there are good ways and bad ways to approach the bar. For instance, at Zachary's restaurant in McLean, a patron can top his salad with tomatoes (35 calories whole) and cucumbers (5 calories in 6 slices) or with far more caloric deep-fried croutons or oil-and-vinegar-marinated eggplant.

Caloric value for croutons differs a great deal, according to Catherine Tarone, a food data researcher at the Department of Agriculture. Commercial varieties can contain anywhere from 5 grams to 16 grams of fat for every 3 1/2 ounces. Since fat contains 9 calories per gram, that can mean a difference of 100 calories.

The popular dips at Zachary's salad bar also vary in calorie potential from high (taramasalata made with red Greek caviar, olive oil and bread) to at least moderate (eggplant salad made with baked eggplant, herbs and "very little oil"). Buttered, toasted pita bread completes the salad bar. But, says restaurant owner George Kontzias, plain toasted pita bread is available; all a customer has to do is ask.

That brings up another point, says Heckman. Customers who watch their weight should ask about the food at the salad bar. Ask if there's oil in the vegetable marinade; ask if the cottage cheese is creamed (240 calories per cup) or low-fat (about 180); ask if there is vinegar, lemon juice or yogurt available to top the salad.

A person who is serious about dieting should choose plain vegetables over creamed or marinated anything. Carrots, green and red peppers, cucumbers, cauliflower, bean sprouts, mushrooms and tomatoes should replace marinated herring, potato salad and macaroni salad. Forgo canned fruits in heavy syrup in favor of fresh ones; eat whole-grain bread, unbuttered.

Follow the same rules at home. Substitute yogurt (about 9 calories per tablespoon) for sour cream (about 26) in creamy dressings. Use chopped meats, cheese, nuts and beans only as garnishes. Remember to use highly caloric vegetable oils and mayonnaise sparingly. A chilled plate, quality ingredients and a beautiful arrangement can make salads aesthetically satisfying even if caloric gratification isn't forthcoming.

Below are two recipes for popular salads and one for dressing which help paint a clear calorie picture. While the salads make delicious meals, they are not necessarily diet fare.

Figures listed beside each ingredient come from the U.S. Department of Agriculture Handbook 8 and the technicians at the Beltsville food data research laboratories. The numbers approximate the calories that each ingredient contributes, allowing you to calculate your own version of the whole salad. Those ingredients with no figures add only neglible calories. In the salad nicoise, no data was available on calories for capers.

FANNIE FARMER'S CHEF SALAD (4 servings) 1 head iceberg lettuce (about 70) 12 radishes, trimmed and sliced (8) 2 stalks celery, julienned (14) 1 1/2 cups salad dressing of choice (bleu cheese, 1,850; french, 1,611; italian, 1,650; thousand island, 2,440) 4 tomatoes, peeled, cut into 6 wedges each (140) 3/4 cup swiss cheese, cut into 1/8-by-1 1/2-inch strips (222) 1 cup cooked ham or tongue, cut in 1/8-by-1 1/2-inch strips (343) 1 cup cooked chicken or turkey, cut into 1/8-by-1 1/2-inch strips (232) 4 hard-cooked eggs, quartered (320) Salt and pepper to taste

Core the lettuce. Take four large, pretty outside leaves and place them around the edges of a large salad bowl. Cut or tear the remaining lettuce into bite-sized pieces, place in the bowl and toss with radishes, celery and half of the dressing. Arrange the tomato wedges around the edges of the lettuce. Combine the cheese, ham and chicken and toss. Spread over the lettuce and vegetables. Place hard-cooked eggs between tomato wedges. Salt and pepper the salad lightly. Spoon the rest of the dressing over the salad. Calories per serving of salad with french dressing: about 740.

JULIA CHILD'S SALAD NICOISE (4 servings) For the vinaigrette: 1 clove garlic Salt 1 tablespoon lemon juice (5) 1 tablespoon wine vinegar (2) 1/2 teaspoon dry mustard 1/2 cup best quality olive oil (954) Freshly ground pepper Fresh or dried herbs, such as basil

For the potato salad: 4 medium-sized boiling potatoes (420) 1 tablespoon finely minced shallot or scallion (12) Salt and pepper 2 to 3 tablespoons chicken stock 2 to 3 tablespoons water

For the salad: 1/2 pound fresh green beans, blanched and chilled (64) 2 ounces anchovies packed in olive oil (100) 2 to 3 tablespoons capers 1 large head boston lettuce, washed, separated into leaves and chilled (23) 4 tomatoes, quartered (140) 3 hard-cooked eggs, peeled and halved or quartered (240) 1/2 cup black olives, preferably imported (80) 7 ounces white tuna (396) 2 to 3 tablespoons chopped parsley (4)

For the sauce, puree the garlic through a press into a small mortar or bowl. Add 1/4 teaspoon of salt and mash vigorously with a pestle or wooden spoon to make a very smooth paste. Beat in a tablespoon each of lemon juice and wine vinegar and the dry mustard. (At this point, you can transfer the ingredients to a jar with a screw top and mix the remaining ingredients by shaking.) Beat in 1/2 cup olive oil gradually. Beat in pepper and a pinch of herbs, taste and correct seasoning.

For the potato salad, scrub the potatoes under water and then cook--by boiling or steaming--until tender, about 20 minutes. Spear each with a fork, cut in bite-size chunks or slice 1/2-inch thick. Toss warm potatoes gently with shallots, salt, freshly ground pepper, chicken stock and water. Allow the potatoes to sit for 4 minutes, tossing once or twice more so they absorb the liquid. Add 1/4 cup of the dressing, toss again and refrigerate.

Clean green beans, trimming ends and brown spots. Slice if desired. Place the beans in boiling water for 5 minutes. Drain, rinse with cold water, drain well and chill.

Toss the lettuce with several spoonfuls of dressing and arrange the leaves around the edge of a large, deep platter. Turn the potato salad into the middle of the leaves. Arrange tomatoes around the potatoes and drizzle with a little dressing. Toss the beans with a spoonful of dressing and arrange around the tomatoes. Distribute eggs, yolk side up, at decorative intervals. Garnish with capers and anchovies. Ring the potatoes with a line of black olives. Break up the tuna and arrange it in the center of the potato salad, or at intervals around the edge. Drizzle with dressing and spoon any leftover dressing over potatoes. Garnish with parsley and serve. Calories per serving: approximately 620.

BLUE CHEESE DRESSING (About 2 cups) 1 cup mayonnaise (1,557) 1 cup plain yogurt (150) 1 tablespoon worcestershire sauce 1/2 small onion, finely chopped or grated (20) 1 clove garlic, minced 2 ounces blue cheese, crumbled (110)

Combine all ingredients. Allow to stand overnight, if possible. Calories per tablespoon: about 60.