Always speak under the assumption that the other person is 'good'. That means not calling them a liar. Instead, ask the questions that you feel would expose the inconsistency. That way they show everyone the point you're trying to make. Also it prevents personal animosity from obscuring the real content you might discover.
Furthermore, it allows a 3rd party who hears the discourse more freedom to make up their mind, whereas they might naturally go against your point of view because it's being pushed down their throat.