Narcissism is a personality disorder where someone believes himself more intelligent or more important than other people around him. Narcissists often lack empathy and fail to see issues from any point of view other than their own and will also distort facts to suit their own agenda. It is sometimes challenging to trust narcissists because they have a tendency to bend the truth. Ensuring that a narcissist remains truthful requires facts and evidence that counteract lies.
Ask the narcissist questions to which you already know the answer. If the narcissist lies to you and you confront him with the real answers, he may be more likely to answer truthfully in the future to avoid getting caught in more lies.
Present evidence to the narcissist that proves she is lying if she continues to deny it. For example, if you have video of the narcissist doing something she says she didn't do, play it for her and then ask her again to start getting truthful responses. The narcissist may reply with more lies to justify the initial lie but this will likely be obvious and can be quickly discredited.
Tell the truth yourself when confronting the narcissist. Exaggerating or hiding a fact yourself for your own purposes will give the narcissist free license to tell even more outrageous lies to further his own agenda. Stick strictly to the truth when dealing with a narcissist and this helps to keep the narcissist on a more truthful track as well.