Psychology is important and it would come in handy. But some people are great actors or believe their lies. If they believe their lies, it poses a problem because they won't show any signs of lying. They would even pass a polygraph.
I'm sure you're looking for a magickal option. Write a spell to reveal the truth to you or bring their true self to the surface so that they act on their thoughts and emotions. But you must remain open to random thoughts, signs, and of course be aware of the person themselves and their slipups. There are truth oils, you can use a candle with the subject's name carved, a picture, words etc. The casting itself is simple, it's the focus and concentration needed that is only for those with patience and dedication. I usually spend at least 30min to 1hr on such a project, continuing to maintain focus and chanting. Keep the words short (4-6 lines) so you can memorize it. After a while I usually fall silent because I've entered a trance state. Once that happens (if that happens for you) you'll know when the spell is "set" and you pull out of the trance slowly and conclude the ritual. The more effort, focus and energy you put in, the better the results.