"Do you remember me telling you we are practicing non-verbal spells, Potter?"
"Yes," said Harry stiffly.
"Yes, sir."
"There's no need to call me "sir" Professor."
The words had escaped him before he knew what he was saying.

There are various ways to call a man a liar. One way is just to scream it at him, which doesn't prove anything. Another is to establish facts by long and patient investigation. Still another way is not to call him a liar at all – let him do it himself.