My wife and I were secret shoppers for a restaurant chain. Not sure what happened, but the program kind of died out (at least for us).
We were given a phone call and asked about specific restaurants for an appointment. We were usually given 3 choices (locations and time of service ... Breakfast, Lunch, Dinner). Once accepted, they sent a form to us through the mail. We'd go to our appointment, pay (our of our own pocket) for the meal (the foods they had on their list), and then send in the questionnaire with the receipt.
A few weeks later we got reimbursed for the meal, up to a specific dollar amount. If the price of the meal went over the allocated amount (because we ordered something extra), that was on us.
DW and I did this for several years and had fun doing it. After a while it became a pain in the neck though, and since it was only the same restaurant chain it became mundane after a while. Then all of a sudden, they just quit calling us.
My sister (in Texas) also did it for several years and she also said, they just quit calling her too. I think with the down-turn of our economy a few years back, the company that did that probably went out of business.
This was our experience. But to make a living doing it ... NO WAY!