In the retail, restaurant and hotel industries good service is what keeps customers coming back. Increasing numbers of U.S. firms are employing a creative new technique to evaluate customer service.

The young woman sitting at the corner table in the restaurant is not a real customer. Nor are the husband and wife who have just checked into hotel room 209, or the young lady trying on jeans in the clothing store, or the man waiting in line for an order of frenchfries at the fast-food restaurant.

All are "mystery shoppers," people employed to pose as customers while really checking on customer service.

Take the young woman in the restaurant for example. Tracey Turgeon, whose firm "DataQuest" employs many such mystery shoppers, says when the young woman leaves, she will answer many questions. "For example, how many minutes did it take for the waiter to initially approach your table? Did they give you eye contact? Were they able to answer questions about the menu?"

Tracey Turgeon says "mystery shopping" has become a $500 million a year business in the United States because businesses have become acutely aware of how important customer service is. "If you have a chain of restaurants and if someone has a bad experience, maybe a very rude server or their food was cold, the typical customer does not take the time to write to management," she said. "Instead they just do not go back. And companies are not aware of this [bad experience], so they will hire our company as a third, unbiased opinion."

Or they might hire Trendsource, the company Mike Causey works for, another "mystery shopper" provider. "We are currently performing just under a quarter-million location visits a year, and we use a key resource of about 6,500 consistently active mystery shoppers and we have 65,000 additional people to draw upon to fill in areas all over the United States and Canada," he said.

As the number of mystery shopping firms have multiplied, so have complaints that they invade worker privacy and generate distrust between management and employees.

But Mr. Causey says most firms are less concerned about those issues than they are about improving service. "Today's market is so highly competitive that companies are looking for that little edge that will help put them one step ahead of their competitors," he said.

Mr. Causey says most firms tell their workers they have hired mystery shoppers, partly in the interest of honesty, and partly to keep them on their toes.