But, even with the original writings of Baha'u'llah, Baha'is have the same problem... How do they "prove" that Baha'u'llah is a manifestation of God and that God is real?
There is no proof that any claim in the history of humanity that any claim of a messianic or 'enlightened' person with a message from the 'Source' some call God(s) or that the "Source' exists.
The writings and history of the life of Baha'u'llah do not have the same problem as the more ancient scriptures, because there is abundant documentation, historical provenance and witnesses to the life and works written by Baha'u'llah.
TB is the only Baha'i I know that just says that there is no proof, only evidence. But other Baha'is continue to say the "proof" is Baha'u'llah.. his writing, his character, and his mission.
I do not and never have claimed any such proof. There is not for any subjective religious belief and belief in the "Source" some call God(s).
For some of us, that's not good enough and complain to Baha'is that their proof and evidence isn't "tangible" proof.
There is no tangible proof for any subjective religious belief nor that for the "Source" some call God(s),
How do you handle that type of question when it comes to supporting the validity of the claims of Baha'u'llah?
Primarily. I simply believe. I do not make outrageous egocentric claims of proof. I have gone into great detail in threads in the past concerning my beliefs. It does center around that IF the 'Source' some call God(s) exists it is a Universal "Source" far beyond the limited beliefs of any religion and the Baha'i Faith. The ancient religions are the most problematic, because they reflect an ancient cultural and tribal view of God, and claim their God is the only God.
The only certainty of the Divine from the human perspectiive is the humility that our beliefs beyond the nature of our physical existence are subjective and in reality we do not know.
Science is the only reliable source of information concerning the physical nature of our existence, and it is always open to change when there is new knowledge.