A Washington woman who had never had a cavity went to a new dentist who told her she needed six fillings. She was flabbergasted, but she let him go ahead ad do the work.

Then she began to have doubts. Why so many cavities all of a sudden? The dentist, she recalls, had seemed overly interested in whether she had dental insurance and whether the forms had been filled out. Maybe the dentist was trying to make an extra buck.

She never did find out, but she never went back to that dentist. And she's never had another cavity.

The moral of the story: Get a second opinion from another dentist before you agree to unusual, extensive or expensive treatment. That's the advice of Joseph P. Cappuccio, professor of oral surgery at the University of Maryland in Baltimore and president of the American Dental Society.

Most of us are usually in awe of our dentist and doctor. They've had the training and they've got the license on the wall. If the dentist says the tooth has to come out, we may cringe, but we usually open wide. But, says Cappuccio, "you should shop for dentistry as you would for anything else."

What should you look for if you go dentist-shopping?

It's difficult to judge what kind of job the dentist is doing with your teeth, says Gregory Lakas, who heads a new low-cost dental clinic sponsored by the Georgetown University School of Dentistry and Some [So Others Might Eat] at Some headquarters at71 O St. N.W. You can't really tell by rubbing your tongue over your new filling; still you know if you're being yanked around, he says.

Look for a dentist who has a light touch, one who gives you jaw as chance to relax during a long session under the drill and one who calms your fears about pain and keeps it at a minimum, he says.

A good dentist has a good "chairside manner," says Cappuccio. Do you like him or her as a person? Is your dentist "gentle," "kind," "reasonable"? And does his or her work hold up?

"If your fillings fall out and you're in pain and you can't eat a decent metal, then you're getting bad service," he says.

Dentists, consumer groups and other dental-health specialists offer these guidelines for evaluating your dentist. Look for a dentist who:

Has an office that looks clean and operates efficiently. You don't want to be No. 20 on the list in a crowded waiting room. "The patient's time is as valuable as the dentist's," says Lakas.

Takes a good medical and dental history on the first appointment and gives a thorough examination of your teeth. He or she should also thoroughly examine your mouth and surrounding muscles, screening for cancer or other disorders. "The dentist is not just a tooth doctor, he's an oral specialist," says Lakas.

Explains what treatment is being recommended, how long it will take and what it will cost, and gives an alternative if you can't afford the recommended treatment.

Makes proper use of such assistants as a hygienist and receptionist. "The dentist who tries to do it all will work himself to death, "says Lakas.

Has a good instruction program for preventing dental problems. The dentist or an assistant should take time to teach the patient how to brush and how to floss properly.

Sends an itemized bill.

In choosing a dentist, a patient should find out what provision has been made for an after-hours or week-end emergency and also how long a wait there is for an appointment. Some dentists can see you in a week; others are booked up for many.

Don't be embarrassed to ask the dentist about his or her fees, advises the American Dental Society. "In fact, most dentists would prefer that the patient open the subject since the patient is aware of his own financial situation and his new dentist is not."

Although many dentists send monthly bills for dental work, there appears to be a trend, particularly among younger dentists, of requesting payment when the work is performed. "You pay cash for your groceries, so why not your dentist?" says one recent dental school graduate, who argues that the rising cost of living makes it difficult for the dentist to cover overhead costs while waiting for patients' payments.

If you're not satisfied with your dentist's work, bring it to his or her attention, advises Michael L. Cady, executive director of the District of Columbia Dental Society. People are hesitant to question their dentist but shouldn't be, he says. If the complaint can't be reconciled, take the matter before your local dental society.

The D.C. society has a 13-member peer review committee which consists of 12 dentists and one consumer. The consumer sits on any panel investigating a patient's complaint, says Cady, who adds: "We've rarely had a resolution that's been acceptable to all." CAPTION: Illustration, no caption