mystery shopper 
noun
A person employed to visit a shop or restaurant incognito in order to assess the quality of the goods or services.At the same time he informed her that a mystery shopper had visited the Lincoln store on Saturday 24 October whilst the applicant was acting manager and the assessment had been a poor one....
- The marketing consultant will act as a mystery shopper to visit all the shortlisted finalists in each category.
- An independent mystery shopper visits all stores entered as part of the judging process.