I used to work for a very large materials testing company - that had labs throughout the UK doing "due diligence" testing on products in shops.

They have people that actually go to large retailers and buy articles and they are tested continually. I got a life from one of the buyers at one of the labs and she mentioned that you get funny looks if you have a shopping trolley filled with, for example, one example of each type of cream cake being sold in that store.