Establishing Purity and Impurity of Private Part

I have doubts whether my wet private part comes into contact with the impure part of my pyjamas when my private part becomes wet after the ghusl or instinja. Can I assume purity because I have no clear physical evidence or proof of urine transferring onto my wet private part? Is certainty of impurity only established by clear physical proof or evidence? An Imam told me if I don’t see the impurity, it doesn’t exist. I have no clear physical evidence or proof of urine on my private part. Jazakallah Khair.