I read where the Nazis had a crack counter agent who consistently smoked out American spies. After the war he was interrogated about his methods.
It came out that he used the same environment, but when the diner asked for pie, he was served it with the wide end first, which is how the Euros ate it. Sure enough, the spy would pivot the plate and start eating from the pointy end, only to look up and see the Gestapo around his table.
A more common clue. Europeans put their knife in one hand and their fork in another. They do not switch hands and the knife and fork remain in their hands throughout the meal. Americans use their dominant hand for both knife and fork and switch them as they work their way through a meal. Dead giveaway.