For uncovering simple deceptions, or disposing of minor lies from minor characters, the Storyteller could just have the mage’s player make a roll and say “Yep, she’s lying” if he succeeds.

In this case, the difficulty is the target’s Willpower. The average person (Willpower 2-4) will be easy to read, while a determined liar (Willpower 5 or more) would present more of a challenge.