I have an old roommate who might be a psychopath

I was “mentored” by this guy for a few years spiritually. It was okay at times, but other times, it was just weird. I try not to think about it too much, but a lot of manipulation and lying happened, yet no one else really believed me when I told them. It's like he's two different people, and no one else really sees that side of him. I was blamed for a lot of our troubles, and now we don't really talk much. I'm worried because he's in a trusted position of leadership and no one suspects anything. They just think I'm attacking his character. How do I know for sure?