Don’t. The thing about people who do things constantly is that they don’t change. If someone lies to you all the time, odds are they’re going to keep lying to you. Personality psychology has taught us that, although people mature, their main personalities stay fairly consistent throughout their life. Unless someone has undergone a serious, life-altering event, it’s safe to assume they haven’t changed. If they constantly lie, just take everything they say with a grain of salt.