Margaret Truman sat in for Dorothy Kilgallen for this telecast and the next while Dorothy was vacationing in Europe.
George and Gracie used a knock on the desk in front of them with their knuckles to indicate "yes" (1) and "no" (2). George and Gracie had a long and successful career in radio (as well as Vaudeville, film and TV) so it seems highly likely that the panelists would have instantly recognized either of their voices had they spoken even with disguised voices. And in fact that was how the deception was revealed; Gracie spoke.