If Bahaullah was not who He claims to be, certainly it was possible to disprove Him. Anyone who has knowledge the knowledge about the claims of Bahaullah would know why.
1. Bahaullah claimed to be infallible: to disprove His claim, all it takes it to prove there is an error in His writings. He has written about 17000 works. In His works, we find statements about previous religions traditions, historical accounts, scientific statements. It is reasonable to say, if there was a mistake in His writings, certainly His enemies would have proved one by now.
2. Bahaullah claimed to be the Promised One of All Religions; the Lord of Day of Resurrection, Return of Christ, the Return of Qaim of Islam, and the Promised One of all Religions: to disprove His claim all it takes, is to prove He does not match the authentic description of those Religions. Anybody who understands Prophecies of Religions would know, that in the Scriptures of Previous Religions, certain signs are given with Regards to the Promised One. For instance, where, when, how... He appears. Thus to disprove Bahaullah, all it takes is to show He does not fulfil the prophecies. It is irrelevant if someone believes in Prophecies or not. They can still demonstrate If Bahaullah does not fulfil them.
3. Bahaullah claimed that all His knowledge is divinely inspired, and He did not go to school to learn that knowledge, neither did He possess any books to learn or quote from, but yet, He knows all that has said or happened on earth: to disprove Bahaullah, all it takes is to show He went to school to learn about religions. History, science and or, He had books to learn or quote from. Certainly if there was such books or schools, it would have been known from historical evidences. The people who went to school in His time, are known in history where they went and what they learned, so, why wouldn't be known about Him, if He indeed went to some school?
4. Bahaullah claimed He knows of future events. For example He predicted down and falls of certain kingdoms and many other things: to disprove Bahaullah, all it takes is to show His prophecies failed.