Lying

Hi Nicholeen,
I recently had a discussion with my 14 year old about his lying. I cannot always tell when he is lying and so it gets confusing when I'm disciplining and he changes the story. It turns into this cycle of me telling him he's lying and him denying it and then I start questioning myself. Anyway, I guess this is a two-part question. How do I know if he's lying and how can I help him be more honest? Thank you!