It certainly is logically sound for the person making the claim to tell us what the evidence is that supports His claim. Who else is going to tell us what the evidence is, the flying spaghetti monster?
Baha'u'llah told us what the evidence is that supports His claims. The Writings are part of the evidence but they are not the first things He told us to look at. His own self and His Revelation are the first things we are to look at and if those are insufficient we are to look at His Writings (the words He hath revealed).
“Say: The first and foremost testimony establishing His truth is His own Self. Next to this testimony is His Revelation. For whoso faileth to recognize either the one or the other He hath established the words He hath revealed as proof of His reality and truth. This is, verily, an evidence of His tender mercy unto men. He hath endowed every soul with the capacity to recognize the signs of God. How could He, otherwise, have fulfilled His testimony unto men, if ye be of them that ponder His Cause in their hearts. He will never deal unjustly with any one, neither will He task a soul beyond its power. He, verily, is the Compassionate, the All-Merciful.” Gleanings From the Writings of Bahá’u’lláh, pp. 105-106
His own Self is who He was, His character (His qualities). That can be determined by reading about Him in books such as the following:
The Revelation of Bahá'u'lláh, Volumes 1-4
His Revelation is what He accomplished (His Mission on earth/ the history of His Cause)
That can be determined by reading about His mission in books such as the following:
God Passes By (1844-1944)
The Revelation of Bahá'u'lláh, Volumes 1-4, which cover the 40 years of His Mission, from 1853-1892.
The words He hath revealed is what He wrote can be found in books that are posted online:
The Works of Bahá'u'lláh