One cannot ever KNOW everything, unless of course, one is a Sammasambuddha.
However, one can make logical conclusions based on what one understands. When one starts understanding the depth of Buddha’s teachings, and the self-consistency, one will start building confidence that a Buddha is a unique human who is born only after long times. That is called building saddha!
One cannot force oneself to have saddha. It comes through UNDERSTANDING. It comes through SELF-CONSISTENCY in the teachings. It comes through one’s own EXPERIENCE in seeing that one’s level of greed, anger, and ignorance diminish with time.
– In fact, it is the last one that is the most important (one’s own EXPERIENCE).
Let me just give a simple example. None of us has been to all the countries on Earth. But do any one of us doubt that any of those countries does not exist?
– How do we know that any given country exists, even if we have no first-hand experience? We know that because there is ample indirect evidence (all self-consistent) for its existence.
Anyway, enough on this topic.
– Just keep in mind that it is dangerous to make derogatory statements regarding a Buddha. There is no need to. It will only serve to block one’s progress. It is OK to have doubts. It is fine to keep an open mind. But making such unnecessary statements can only be harmful to oneself.