You asked how you could stop assuming someone is lying to you.
I had to 'relearn' my approach to people. Not just about lying, but other situations that left me feeling vulnerable.
"White lies", I feel everyone does that, to some extent. I like to think of it as exaggeration more than lying.
What came to mind in the situation about snake boy is to learn to ask questions first. Like, ask him who he contacted to find out if it was legal to have the snake. Instead of assuming he's being dishonest. His answers would have given you a hint. The keyword is to LEARN.
I understand about being lied to. And sometimes it was so blatant that anyone with eyes and ears would know it was a lie.
I have 'learned' to trust MYSELF. If my gut is telling me one thing, then I simply need to listen to my gut. Some lies do not matter in the bigger picture of my life. A random encounter with someone that I will never see again. Doesn't matter.
Someone I am close to, love or have a relationship with, yes. Those lies matter.
So I learned to ask questions instead of jumping to conclusions. Not aggressive and combative questions, but more to let the person know I feel they were not being honest.
It became important for me to understand that people are lying to themselves, not necessarily TO me.
Trust is and always will be an issue for me. But learning to trust myself has helped me to not assume the other person is wrong or lying.
Takes time to build this new life when trust has been betrayed time and again. I respect you for wanting this to change in your life. Wishing you success with your journey.