As Brenda Lee of Santa Clara approaches the entrance of Vin Santo Ristorante in San Jose, she notices that the sidewalk is nicely swept clean and that the hostess greets her with a warm smile.

Shown to her table, Lee purposely takes a seat facing into the center of the restaurant. As she converses with her dining companion, her eyes dart around the room, taking in every last detail.

Nibbling smoked salmon, prosciutto and tomato-basil crostini, she holds the last morsel in her fingers when a busboy comes by to clear the table -- a little too soon in her book. After the waiter sets down her entree of grilled trout, he doesn't check back for a full five minutes -- quite a bit longer than the two minutes she considers acceptable.

Lee is finicky to a fault. She has to be. She is a ``mystery diner,'' enlisted by restaurants to dine incognito, then give feedback on service, food and decor to help them improve their operations.

This night she was merely demonstrating to a Mercury News food writer how she conducts her work. Had this been a real evaluation, Lee would have driven home afterward, then stayed up for the next two hours to fill out a seven- to 10-page online report about her dining experience.

For foodies who dream of being restaurant reviewers, mystery dining can be a fantasy come true. But as anyone who has done it will attest, it also can be a lot of work. It takes painstaking concentration and recollection, particularly because mystery diners are forbidden from jotting down notes at the table.

Moreover, Lee does all this without pay. Her only compensation is the restaurant meal she gets to eat.

``It's fun, and I enjoy taking family members, friends and even dates,'' says Lee, 47, who mystery-dines three times a month. ``But I also have to remember that this is my weekend. And I have to build in the time to do this.''

All over the world, mystery shoppers like Lee surreptitiously infiltrate and critique almost every industry, including quick-service and high-end restaurants, hotels and resorts, banks, apartments, convenience stores, self-storage facilities, health care organizations, health clubs, grocery stores, and chain and specialty retailers.

Mystery shopping has been around for decades. The industry, which had an estimated value of almost $600 million in the United States in 2004, is growing at 11.5 percent annually, according to a report by the Dallas-based Mystery Shopping Providers Association. With more than 260 member companies worldwide, including 21 in California, the 8-year-old MSPA is the largest professional trade association that works to improve service quality using anonymous sources.

``Mystery diners look at details the average customer doesn't pay attention to,'' says Brad Leone, an MSPA spokesman. ``Did the server meet you in two minutes? Were the specials mentioned? Is the coffee hot enough? They provide a clear snapshot of the business.''

Lee, a technical writer and a part-time copy-editing instructor at San Jose State University, started moonlighting as a mystery diner or evaluator three years ago, just because she loves to eat out.

She works for EyeSpy Critiquing and Consulting in San Jose, which unlike most mystery shopping companies that review a broad range of businesses, is one of the few that concentrates solely on restaurants, as well as hotels that have restaurants.

Founded five years ago, EyeSpy has 103 restaurant clients, including Aqua in San Francisco, the Lark Creek Restaurant Group in the Bay Area, Straits restaurants in the Bay Area, the Village Pub in Woodside, Steamers Grillhouse in Los Gatos, Willow Street Wood-Fired Pizza restaurants in the South Bay and Armadillo Willy's Bay Area locations.

Its volunteer evaluators number 2,300 in Northern California, 300 in Los Angeles, 300 in New York and 30 in Kansas City. Its waiting list for potential new evaluators already tops 300.

Restaurants pay EyeSpy $75 per evaluation, with most restaurants opting for five evaluations per month. Restaurants also reimburse the evaluator for the cost of each meal, typically the equivalent of two drinks, one appetizer, two entrees and one dessert.

Restaurants do this to stay on their toes: to make sure employees are performing up to snuff and that diners are experiencing the establishment as intended. While newspaper and magazine restaurant reviewers do the same to some extent, EyeSpy mystery diners scrutinize specific criteria that can be customized for each restaurant.

``Some restaurants do put out customer comment cards or offer a gift certificate if customers fill out a survey online,'' says EyeSpy founder Mistie LoNardo, 33, a former South Bay regional restaurant manager. ``But their customers might not look at the learning points that we do. For instance, at Willow Street, they really want their servers to say, `Please enjoy our homemade honey wheat bread' as they set it on the table. Customers might not know that.''

Some restaurants specifically request to have a young-looking evaluator order a drink at the bar to see if bartenders are asking for ID as they should, LoNardo says. One San Francisco restaurant wants the evaluator to order a dish with an accompaniment it doesn't normally have to see how the staff handles the special request. And one Los Angeles restaurant wants the evaluator not only to eat a meal, but also to ride the mechanical bull, because it considers that part of the full dining experience.

The feedback can make a big difference. Some restaurants link staff bonuses to the evaluations. Or give an added perk to the locale that receives the highest score in the chain. Or even $100 to a server who gets a 100-point score.

Aqua, which has EyeSpy perform two evaluations a month, still is working to fine-tune the presentation of its souffle, which a mystery diner found a bit awkward, says Jean-Claude Persais, vice president of operations for the restaurant. The evaluator noted that after the waiter punctured the airy dessert with a spoon to pour in the sauce, the gloppy spoon was then left on a small plate at the table.

At Willow Street in the Westgate Center in San Jose, regional manager Nancy Reineking found out just how badly the bathrooms needed a makeover when comment after comment from EyeSpy evaluators gave them thumbs-down.

``The color was dated. It was just mediocre-looking and not nice,'' Reineking says. ``We remodeled them completely. Now we get comments about how nice they are.''

Many of EyeSpy's evaluators are chefs, bartenders and even clients. But EyeSpy won't send them to evaluate a competing establishment, LoNardo says. All evaluators must adhere to confidentiality requirements: They can tell friends and family that they've visited a particular restaurant, but they can't reveal what they thought of it.

Evaluators are trained to discern good service, to look for unsightly paint cracks in walls and to gauge a proper liquor pour for a mixed drink, LoNardo says. Beginner evaluators start with reviewing casual, counter-service spots, then work their way up to more exclusive establishments.

The more high-end the restaurant, the longer the evaluation form. Some can take as long as five hours to complete, with not only yes-and-no questions but extensive essay-like ones. Evaluators must complete the form within 24 hours of their restaurant visit, so it can be turned over to the restaurant within three days.

While dining out on an evaluation, mystery diners are admonished to avoid saying words such as ``observe'' or ``evaluate,'' to not discuss service or food quality, to not ask excessive questions and to not say or do anything that would draw too much attention to themselves. Even so, some evaluators must be let go after a couple of tries, LoNardo says, because their writing or recall skills just aren't up to par.

Husband and wife Bob and Lin Korns of Los Gatos, both 61, have been evaluators for EyeSpy for 3 1/2 years. As a special education teacher, Lin Korns says, the ability to focus sharply comes naturally to her. Plus, as a gourmet cook who has traveled the world, she finds it easy to distinguish good food from bad.

Once, she described a Caesar salad dressing at a restaurant as so thick it was like the Exxon Valdez spill. Another time, she recommended a hostess stand be reconfigured because customers entering the restaurant were greeted by the reservationist's back.

``It's really enlightened us on what it takes to run a restaurant,'' Lin Korns says. ``It takes so much more than what people think. It's a very complex and complicated system.''

So much so, she says, she'd never want to own a restaurant herself.

She's content to live a shadowy life of mystery instead.

IF YOU'RE INTERESTED

For more information about EyeSpy Critiquing & Consulting, go to www.theeyespy.com or call (408) 292-1612.