Many service-oriented industries, such as grocery stores, retail stores, banks and casinos, wish to evaluate their customer service levels, but they find it difficult to be completely objective. Instead, many of these businesses turn to a mystery shopper, a trained and independent service evaluator who visits the facility as a typical customer. Detailed notes of his or her experiences are later sent to managers for careful examination and discussion with the staff. A mystery shopper is usually encouraged to be objective and present both negative and positive opinions based on industry standards.

A mystery shopper may simply walk into a restaurant and order a meal, or she may be assigned a specific challenge to test the flexibility of customer service. A mystery shopper may inform the hostess of a restaurant that she is allergic to salt, for example. The restaurant's response to such an unusual request may be part of the test. A mystery shopper may also act hard to please or attempt to return merchandise without proper receipts. This can help managers expose holes in a security system or discover employees who are rude to difficult customers.

Professional mystery shopping companies provide different levels of quality control analysis, from a simple sales transaction to an in-depth evaluation of the premises, staff and merchandise. Often, a part-time mystery shopper is asked to purchase a specific product, such as a hamburger from a fast food restaurant, then fill out a detailed form concerning the experience. The mystery shopper may be paid an hourly wage to walk through a store or merely reimbursed for any purchases made and evaluated.

Not all mystery shopper transactions involve a purchase, however. Sometimes, the time between a shopper's entry and his recognition by the staff is noted, or the mystery shopper may ask a sales associate about the merchandise. Managers may also want to know about the cleanliness of a particular store or any suspicious behavior by employees or customers.

Becoming a mystery shopper is often a matter of registering with a legitimate mystery shopping agency and waiting for assignments. A number of fraudulent mystery shopper programs have appeared both in online and print advertising, so be especially cautious when answering any notice for mystery shoppers wanted. Legitimate companies do not charge a sign-up fee, nor do they ask for personal financial information during the application process.