What is the ruling regarding someone who doubts whether a najāsa (ritual impurity) has afflicted their garment or body, and are they obligated to inspect it? And what is the ruling if they find something and doubt whether it is a najāsa?

There is no need to inspect if a person doubts whether impurity has afflicted their body or garment. It is sufficient to act upon the original principle, which is that no impurity has afflicted any part of their body or garment, and this principle should not be abandoned except with certainty. If one doubts the impurity of a specific item, the original principle is its purity, and impurity is not to be ruled merely based on doubt. Everyone should beware of obsessive whispers (waswas (definition)) and not open the door to them, because succumbing to them leads to great evil.