How do you know they are from God? Many writings make that claim.
What writing includes the logical proof for God's existence?
Some Answered Question is available here in PDF form: (words of Bahaullah's son)
http://www.bahai.org/library/authoritative-texts/abdul-baha/some-answered-questions/
Just like we have been given eyes to see the physical world we have also been given insight to be able to discover God. So we have an innate ability to recognise God.
Baha'u'llah talks here about ways we can know truth.
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. (Baha'u'llah)