People are never as honest as when they lie. Ultimately, people might fail when they try to honestly portray their character. They might be unaware of some aspects of their own personality. But when they lie, they reveal much more important things about themselves.
See how they portray themselves in their lies, and that is more important to know their character than its naked reality. People, in their lies, construct not who they are but who they want to be, themselves as they want it to be perceived. They reveal their values, their fantasies, their unfulfilled desires. Listen to people when their honest and you will have data, listen to them when they lie and you peek through their aspirations, goals, and regrets.
Do they lie about how rich they are? Then it is wealth that they seek. Do they lie about the books they’ve read and pretend to know what they really don’t? It’s knowledge and intellect they seek. Do they lie about their sex life? Or their adventures? Or people who love them, or hate them?
Listen to people lie. That’s all.