A spy recognizes another spy. Maybe not immediately, maybe it takes a while for the realization to hit, but eventually... every spy, if they're good enough, knows how to identify another.
Walter can see the moment when Molokov realizes, a flicker in his eyes, and they've been in the same room less than five minutes. The other members of the Soviet and American delegations don't matter at the moment - they meet eyes, and they both smile, and they know.
"Mr. de Courcey," Molokov says. Walter smiles and extends a hand.
"Mr. Molokov."
No words exchanged, just a handshake that lasts perhaps a second too long, but that's all that's necessary. A good spy always knows another. And Molokov, Walter can already tell, is one of the best.