By their backside in their hosts' faces, apparently:
"As the agency’s leading disguise specialists, the Mendezes spent decades creating false identities for American spies
Often, the biggest challenge facing the Mendezes wasn’t coming up with fake mustaches and plausible covers — it was teaching agents to act less, well, American.
To wit: Europeans stand up straight in elevators, with their weight on both feet. Americans slouch, leaning against walls and railings. In movie theaters, Americans walk though crowded rows facing the screen; by contrast, Europeans face the audience, because they consider sticking their backsides in their fellow patrons’ faces to be rude.
Mrs. Mendez said contemporary American agents have to be taught to smoke. Mr. Mendez added that even the American habit of daily showering can be a giveaway.
“In Moscow, we made sure agents had the right amount of garlic, vodka and plain old B.O."