A special method for test marketing is use of spy mirrors. A test shop is mounted with a spy mirror. Customers are shopping as usual and behind the mirror observers are taking notes of consumer behaviour and/or video recorded. After ending the shopping the customer is interviewed. She/he is asked why they had a specific behaviour concerning a special product group.
Spy mirror is a technology to go behind customer behaviour. Spy mirrors can have the problem that the customers do not accept having "a big brother watching you". Also legislation can give you limitations for the use of such technology.
A practical solution can be only using the video observations already done in retail shops in order to prevent against shoplifters. The cameras can also be focused on a product group.