Question: How can I find legitimate mystery-shopping jobs without fear of being scammed?

Answer: It's wise to be leery. As mystery shopping becomes an increasingly popular way to earn extra income, online scams and questionable practices have emerged, targeting consumers hungry for such work.

Mystery shoppers are hired by independent "mystery-shop" firms to do undercover evaluations of customer-service-oriented businesses such as stores, restaurants and banks, and sometimes to evaluate Web sites and telephone-customer service. The firms that hire mystery shoppers keep databases of shoppers available nationwide, and when an assignment arises they contact interested shoppers in that region. The enlisted mystery shoppers survey the businesses and fill out detailed questionnaires about everything from the cleanliness of the floor to the helpfulness of the store clerk. Most assignments pay $10 to $30, though some pay more.

Unfortunately, it's gotten harder to scout out legitimate jobs amid the numerous shady practices. Several Web sites and spam emails, for instance, claim to put your name in databases used by mystery-shop firms for a one-time fee, usually ranging between $20 and $100.

In truth, you needn't pay a dime to sign up with mystery-shop firms and it's generally not the lucrative career these services suggest, says Ron Welty, chief executive of IntelliShop, a Perrysburg, Ohio, firm that hires people to mystery shop for 115 clients across the U.S. and Canada. "I've yet to get any kinds of lists from those kinds of companies," Mr. Welty adds.

Some mystery shoppers can earn $25,000 a year or more, though such shoppers usually sign up with several firms and do dozens of assignments a week. "It becomes a matter of submitting good-quality work and being very reliable," Mr. Welty says. Most mystery shoppers make less than a few hundred dollars a week.

Thankfully, there's an easy place to get on legitimate mystery-shopping firms' lists. The Mystery Shopping Providers Association carefully vets its 200 or so member mystery-shop firms. Members cannot charge any fees and must be in business at least two years to join. To find member firms and potential assignments in your area, go to www.mysteryshop.org/shoppers and click on "Search For Assignments Here."