If you see Maryellen Boissonnault standing at a store counter or eating at a restaurant, you'd assume she is a typical shopper. But she's working undercover to ferret out how well the employees are doing their jobs, and how the business could be improved.

If you see her glance at her watch to time a transaction, her move will look so casual you won't think anything of it. If she jots a couple of words in a small notebook, you probably won't even see it happen. She's that good.

Maryellen has carried out about 500 such assignments for mystery shopping companies that report the results to the managers of the targeted businesses. As far as she knows, her cover has never been penetrated at any store, restaurant, hotel, or automobile service shop she has visited.

"I like it because I can make my own hours," she says. "I work when I want to work and I don't work when I don't want to work. And it's always a nice treat to go out to eat and have someone else reimburse you."

All sane people like to shop, eat and get paid. Should you follow in Maryellen's footsteps? Let's look inside her world.

Mystery Money
Maryellen, a retired high school teacher, takes about three assignments a week to supplement her income. "For a restaurant assignment it might be reimbursement for the meal plus $6 or $8. But reimbursement for an oil change is about a $39 value, and I've stayed [free] at a hotel now and again."

Rushton McGarr, a vice president of the parent company of Shop'n Chek, one of the companies Maryellen works for, says it's rare for anyone to make a living at mystery shopping, and the few who do are very organized people.He said compensation varies, but a 15-minute job might earn a shopper reimbursement plus $10 or so.

How it Works
Mystery shopping requires Maryellen to report on a number of specific details of each experience. At a restaurant, how long it takes an employee to greet her, and how long does it take to get her food? Were the table, chairs and menu clean? Did the server refill her beverage without being asked?

When she leaves, she fills out the paperwork for her report right away, sitting in her car. That takes 20 minutes to a half-hour. "When I first started out it took longer, now I'm used to it," she says.

What it Takes
Maryellen learned of mystery shopping in July 2003. She began applying for assignments on the Web sites of companies that belong to the Mystery Shopping Providers Association (mysteryshop.org ), which promotes ethical standards in the shopping industry. She got her first assignment about a month later.

Shoppers also can become certified through the association for $15 to $99. The association makes it clear that companies will hire shoppers without the certification, but it might give shoppers a leg up with some companies. Maryellen didn't bother becoming certified, and it hasn't slowed her down. She currently gets assignments from 17 companies, all of them among the association's 260 members.

What to Avoid
Maryellen steers shoppers away from companies that do not belong to the association, and warned that some non-affiliated companies will ask mystery shoppers for money.

"You never pay to be a mystery shopper. They pay you," she stresses. She found out that two people she knows were paying for the privilege of mystery shopping, and steered them right. Now they do assignments for association-member companies. Maryellen said a good mystery shopper is "very, very observant," able to follow directions, and willing to handle the paperwork. "If you're not intimidated by that and you can write a complete sentence, you'll be okay," she says.

Shop'n Chek hands out some 55,000 mystery shopping assignments each month, and McGarr said the company prizes reliability and attention to detail in its shoppers. Maryellen said she would recommend mystery shopping to "anyone who wants to supplement their income."