I discover dubiety in myself. One symptom is that I ask a certain question often: "Is what I just thought generally true, or merely locally true (although general across my experience)?"
It's a sampling problem. How do I know that I have enough samples, and have weighted them properly, to be representative of a class?
This is not a big problem in studies that look at relatively simple and interchangeable objects, like widgets or waves. It is a very big problem knowing and describing complex and varied subjects, like women or turbulence.
It's much worse in philosophy. "The study of the sui generis," as I've heard it called, necessarily has real problems with this very issue--when do I know what I know is enough to know things?