You almost always assume that a person is telling you the truth, so when you think you've caught someone in a lie, it's easy to feel betrayed. You may hesitate to call someone a liar -- especially if you think there's a chance that you're wrong -- but it's important to let her know that you know the truth. As an adult, you can't really resort to chanting the "pants on fire" rhyme -- you have to handle the situation in a respectable fashion.
Collect evidence that supports your view. If possible, you want to be able to tell the suspected liar why you think she told a lie. Validate your position. Make sure that you are absolutely right before you accuse someone of lying. Your false accusation could destroy your friendship or business relationship.
Invite the person to talk. This should be during a time when there are no other distractions, such as a weekend afternoon or during dinner.
Ask the person to clarify what she previously said. This gives her a chance to come confess to the lying, though she's likely to continue with the lie.
Tell the person, "I know you're lying." Present any information that you have that proves that she was lying. Tell her how her lies have made you feel.
Maintain your position if you disagree. Some people will instantly admit a lie when confronted; but others will stay the course. If you're sure that you're right, don't back down. Do not provoke a physical confrontation. Ask her why she spoke falsely. Use open-end questions that require elaboration in her answer, such as "Why did you say that when you knew it was wrong?"