There is an outtake from Part I, which should be on your bonus disk, that shows Sonny getting a call in his father's office from his man in the phone company, confirming some numbers Sonny asked him to check out. The call confirms Paulie as the traitor. Sonny says to Michael, "Whose brains do we blow out? Clemenza's or Paulie's?" Michael says, "Not Clemenza." Sonny says (to Tessio)"See? Bright college kid. It was Paulie..."