When Hogan and his men do the Nazi salute to Carter, who is impersonating Hitler, LeBeau has his hand made into a fist and two fingers crossed.
When Newkirk introduces Carter in his guise as Hitler, he calls him Adolph Schicklgruber. That had been the original family name of Hitler's, as his father had been an illegitimate child and did not assume the Hitler surname until his mother got married years later and then took on the Hitler name.
When Foster assumes a German identity he calls himself Major Hauptmann. Hauptmann is the German name for the Army rank for Captain, so he is calling himself Major Captain.
The title comes from the question that the host of the TV game show To Tell the Truth (1956) asked the impostors to reveal to the panel and to the television audience who is the real guest and who are the impostors.
Even though Carter, impersonating Hitler, parts his hair on the wrong side (his right side), he does indeed "redeem" himself in a later episode, parting his hair on his left side.