How to Prove to Someone That You Are Not a Liar
To prove that you're not a liar, stay calm, be confident in your story, and show them any evidence you have. Although you might feel angry or upset that someone doesn't believe you, take some deep breaths to relax, since getting emotional can make you look guilty. Make eye contact with the person, since looking away is something liars often do. Then, give the person any evidence you have to support your story, such as a shop receipt. For example, if your girlfriend accuses you of being out with someone last night but you were studying in a coffee shop, show her the receipt for a drink you bought. Alternatively, ask a friend you were with at the time to confirm that you were there.
 Remain calm. When you are accused of something, especially if you are telling the truth, your first reaction may be to get angry or defensive. Getting overly excited or frustrated, while a fair response to an accusation, can actually make the other person believe the accusation is true. Take a few deep breaths , and remember that you are trying to resolve the situation and preserve your relationship with this person, not make things worse.  Make your claim, with confidence. With some people, the only way to prove you are not a liar is to not prove it. In the end, you can't force another person to believe something different than what they are already committed to believing. However, you may be able to shake the person's confidence in his opinion by stating your own truth with confidence. Stand firm with your feet planted hip distance apart. Roll your shoulders back and lift your chest. Raise your chin and make eye contact briefly (about 5 to 7 seconds). Tell the person, "I see that you are resigned to believe what you think happened, but I know what happened. I did not do this, and I'm sorry you can't see that." Refrain from saying anything more, as overly explaining can be misconstrued as lying. Ask the person what you can do to regain trust. When it's clear that you cannot convince this person of your honesty, ask for more information about why she seems to distrust you. Equipped with this information you can strive to be more trustworthy in the future. Whatever the person tells you should be used to change your behaviors so that you do not have trust issues in your relationships moving forward. Provide evidence of what happened. The best way to prove to someone that you are not lying is to offer the person evidence that directly contradicts the claim. If you can think of any way to demonstrate your truthfulness, do so by backing up your statements with credible proof. However, in some cases, it may be impossible to show evidence that you're not lying. For example, you can show evidence by producing paperwork, such as time-stamps or receipts. If your girlfriend accuses you of being out with someone else on Tuesday night, it can prove your innocence by showing a time-stamp that you were using a computer in the college library until 3am. Use social proof. Social proof is the concept of using others' opinions or beliefs to influence a person's decisions or behaviors. If there isn't any physical evidence, you may be able to convince the person by employing someone (or several people) who can testify to your innocence. For example, if your mom feels certain that you were not at a friend's house, but had snuck out to a party, you can back up your story by asking your friend's parents to testify on your behalf. If two adults--and seemingly trustworthy people--say that you were definitely at their house, your mom is more likely to believe you. Repeat the chain of events in reverse order. A method often employed by law enforcement is the process of making a suspect repeat a chain of events in reverse to see if this version of a story will be consistent with the original version. Tell the person to ask you about what happened by starting at the end. If you are lying, you are more likely to screw it up or show inconsistencies by telling the story backwards. Make a promise. Explain to the person who is accusing you of being a liar: "I wouldn't lie to you, I promise." Hopefully, if the person has any faith in you, or you haven't already damaged the trust previously, she will believe that you are being honest. If you have a particularly spotless history with this person, you could add to the promise by offering them examples of ways you have been honest and trustworthy in the past. For example, you might say "I have never lied to you. Remember, that time when everyone else wasn't being straightforward with you, and I did?" Be careful with this method, though, because some people will think you are manipulating them by making a false promise. Keep your words simple. Research shows that liars often talk in more complex sentences than people who are telling the truth. Even if you are telling the truth, speaking in complex sentences can make it seem like you are covering something up or being evasive. Strive for clear, simple, and concrete sentences with a moderate level of detail. You might tell your girlfriend, "No, I was not with anyone last night. I was at the computer lab on campus. I can show you my time-stamp." rather than "Oh, what are you talking about? I would never do anything like that to you. I love you so much. Who could I have been with? Why don't you trust me?" All the extra questions and miscellaneous information sheds doubt on your honesty. Avoid repeating questions before answering them. Another science-tested characteristic of liars is the tendency to repeat a questions before providing an answer. Avoid this at all costs. If you didn't clearly hear a question ask the person to repeat it himself. Repeating it from your own mouth makes it seem like you are delaying your response so you can think up an answer. An example of this happening in action could be demonstrated in this way: Your partner asks "Where were you last night?" If you say "Where was I last night?", pause, and then add "I was in the computer lab doing work," this seems less than honest because of the need to hesitate before providing your answer.  Examine past lies. If you have been known to tell a fib or two in the past, you need to carefully examine this behavior and try to learn from it. Even if you are telling the truth in this situation, you may have developed a reputation as a liar. Look at other times you have lied or been caught in a lie. Brainstorm ways you could have prevented the lies and been honest from the start. Once you have made the decision to stop lying, you help convince others of your truthfulness by sharing details about your life upfront. By doing this, you remove the need to be questioned, and therefore, others don't have to wonder if your answers are lies.  Be open. If you have trouble opening up to others, this may explain why someone is doubtful of your honesty. Learn how to be more open and honest in your close relationships by following these tips: Be more accepting of others and yourself. When you are constantly worried about what others will say or how they will look at you, you may lie just to say what you think they want to hear. Trust the people you love to care about you despite your flaws. Don't put off important conversations. Schedule time to talk about important matters when both parties are free to speak and think clearly. Live a life according to your values. Live your own truth, and then you don't have a need to lie. Embrace constructive criticism. We may lie because we don't want others to judge or criticize us for our faults. When you are able to see criticism as a tool to improve, you won't have to rely on untruths to get you by. Make appropriate eye contact. Evading others' eyes is often seen as a sign of dishonesty. Eye contact is an effective way to show honesty and sincerity. You are also able to forge a more intimate connection with another person by staring into her eyes. Just be sure not to overdo it. Too little eye contact can seem evasive, but too much can seem threatening or intimidating. Experts suggest maintaining eye contact longer for one-on-one conversations, about 7 to 10 seconds. The amount of time making eye contact and looking away should be around 30 to 60 percent --it's okay to make more eye contact when you are listening and less when you are talking. Be aware of what your body is saying. When we are telling the truth, we are more comfortable. In the midst of lying, we may be more uncomfortable. A liar may demonstrate closed off and unfriendly body language. On the other hand, a truth-teller will orient towards the other person, smile, nod, and uncross their arms and legs. Sometimes, even when you are telling the truth, you may put your guard up because someone is questioning your behavior or motives. However, the other person can wrongly interpret this as being defensive and think you're lying. Take a deep breath and try to relax with the knowledge that, no matter what, you know that your words are honest. Don't allow your anxiety about being questioned undermine your ability to convey the truth. Be mindful of your body language. Avoid crossed arms/legs or clinched fists to demonstrate a more friendly demeanor even when you are sort of irritated about being questioned. 
 Remain calm.  Make your claim, with confidence. Ask the person what you can do to regain trust. Provide evidence of what happened. Use social proof. Repeat the chain of events in reverse order. Make a promise. Keep your words simple. Avoid repeating questions before answering them.  Examine past lies.  Be open. Make appropriate eye contact. Be aware of what your body is saying. 