The tough economy means companies are having to take extra steps to make sure their customers are happy, so more are trying out mystery shopping.
Demand has gone up by 40% in the past two years, the Mystery Shopping Providers Association says, and the industry is worth an estimated £70m a year.
Mystery shopping started in the 1940s, and the association says well over a million assessments are carried out every year.
Each one takes up to 20 minutes, and the secret shopper can be paid up to £15 a go.
We decided to go undercover and try it for ourselves. One mystery shopper, who has been doing it for years as a part-time job, gave us some tips. Unsurprisingly she asked to remain anonymous.
"You're looking out for if the store is clean, the staff are friendly, and if they approach you," she says.
"If you ask a question, do they take you to the relevant section? Do they ask you open questions, and try to sell you as much as they can?"
With that in mind, we headed to the Llantrisant branch of Leekes, a department store chain based mostly in Wales.
The company knew we were coming, but they had no idea when.
It was a mixed experience. We had to search for someone to help us in the homeware section. But once we had found someone they were friendly and approachable.
In other departments, staff were just hanging around, chatting to each other. But once we asked, they did their best to get a sale.
There were some examples of excellent customer service too.
Some sales assistants did everything they could to answer our questions as helpfully as possible.
But Steve Hurst from the Mystery Shoppers Providers Association says the actual store visit is only part of the job.
"It's just as important what we, and the company itself does with the information. We put it all into a database, and can break it down from the bigger picture across the chain, to each individual store."
Companies pay about £40 each visit, which means nationwide chains can pay hefty bills.
It is difficult to say with any certainty what kind of return businesses get on that investment, Mr Hurst says, but he insists it is a cost-effective and unbiased research tool.
Fake and outdated
Not everyone agrees though. Jeremy Michael is from customer research company SMG, and used to work in mystery shopping. He now thinks it is outdated.
"Years ago, it was the best way to find out what was going on in a store, but now you've got much more modern methods," he says.
"Clients like Superdrug and Pets At Home have moved away from asking fake mystery shoppers, to asking genuine customers what they thought and how they can improve their experience."
But Leekes, who use mystery shoppers, say the feedback helps them.
"There are some positives in the way that staff dealt with you, but some negatives in terms of not being available when you were looking for assistance," says Peter Martin, store operations director at Leekes.
He says that the company will be able to learn from it, and fix any problems.
For them, mystery shopping is not about catching staff doing something wrong, but giving them a chance to show off their skills and training.