Duncan Hines (yes, of cake mix fame, but he was a critic before he began endorsing products) benefited from the services of so-called "dinner detectives" for his book Adventures in Good Eating (1936; several subsequent editions). These were a number of friends and colleagues who volunteered to keep tabs on the recommended restaurants listed in Hines's book and report back to Hines as well as suggest new restaurants for inclusion in subsequent editions. The dinner detectives included Forrest A. MacGibbon of Marshall Field; Frank J. Wiffle, secretary of the National Restaurant Association; broadcaster Mary Margaret McBride; and Metropolitan Opera star Lawrence Tibbett.
See Louis Hatchett's Duncan Hines: The Man Behind the Cake Mix (2001) for further details (pp. 60-61).
About the photo: Celebrated baritone and dinner detective Lawrence Tibbett. Library of Congress, Prints and Photographs Division reproduction number LC-DIG-ggbain-36506.