I don't have a direct answer for you, but I may have found the article you read while in college.

Pattinson, Juliette (2010) 'Passing unnoticed in a French crowd' : the passing performances of British SOE agents in Occupied France. National Identities, 12 (3). pp. 291-308.

The quote from the article in question:

Agents were advised to find out about “unprocurable articles (e.g. danger of ordering wrong drinks or cigarettes)” (National Archives, HS 7/55) and their table manners were observed to ascertain the degree of their familiarity with French customs and to discover whether they had any British mannerisms that might betray their identity. Derrick Duesbury recollected that Security section personnel “watch[ed] how they used knives and forks. Whether they lift them like this, [mimes French manner] if they are going to France” (interview, 1999). This was because, as Despaigne observed, “the knife and fork are handled in different ways in France than they are in England . . . they cut their meat first and then change over” (IWM SA, 9925). Cyril Watney noted that table manners were culturally differentiated: “If I’m drinking soup here in England, I’d put my soupspoon there and I’d drink the soup like that. But no, not in France. In France, you pick up the whole thing and knock it back . . . There are lots of different things that you’ve got to know” (interview, 2000).

Pattinson is quoting there from interviews conducted with sources listed as "Derrick Duesbury, (pseudonym) Security section" and "Cyril Watney, agent". as well as an interview of "Harry Despaigne, agent" held on file by the Imperial War Museum Sound Archive.

As to if there are other sources that can help confirm this, I honestly don't know, but this would be a starting place.