Simple: Their lips are moving. Which is confirmed by Kinsley:
The Big Apple: “A gaffe is when a politician tells the truth” (Kinsley gaffe): "'A ‘gaffe’ is the opposite of a ‘lie’: It’s when a politician tells the truth,” wrote political pundit Michael Kinsley in 1984. It’s become known as Kinsley’s Law of Gaffes or a Kinsley Gaffe: “A gaffe is when a politician (accidentally) tells the truth.”"