I'm pretty sure that we all agree that The Bible was written by men. So here's the question: if the Bible was written by men, are Christians believing in God, or the writings of men who claim that the "Word" is from God?
If the answer is the former of the two, which I undoubtedly know the majority of responder will pick, how do you figure that the Bible is really from God?
Yes, the bible was written by men - in fact, the bible was written over roughly a 1500 year span (from ~1400 B.C to ~A.D. 100), 66 different books, letters, prophecies, poems, etc. by men from diverse walks of life, kings, peasants, fishermen, poets, scholars, etc., from different generations, different times/eras, on different continents (Asia, Africa, Europe), and in different languages (Hebrew, Greek, Aramaic).
It is the most popular book in the history of the world - more copies of the bible have been purchased than any other book, ever (though arguably the number of purchases doesn't equate to how often it's actually read), and the bible is the most translated book in the world - into more languages than any other book, ever.
Christians believe the men who wrote the bible did so as they were inspired by God to write. In this sense, we believe God is the actual author of the bible.
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be competent, equipped for every good work. - 2 Timothy 3:16
While this may be difficult for some to accept, the notion of "inspiration" drives ALL authors to write what they do and it is the veracity of that inspiration that leads one to believe what the author has written, whether it is words of their own smithing, or from someone else.
As to that veracity, one cannot - indeed SHOULD not come to a belief in anything they have not themselves actually read. For some, the mere notion that a book has been inspired by God is foolishness because they don't even believe God exists - which is a first premise for them, and a seemingly insurmountable hurdle to boot.
Christians arguably "know" - but certainly "believe" God wrote the bible on the basis of their having read (and studied) it. Such belief, or faith in God's authorship is, imo, primarily a function of what they read and how what they read touched them.
For the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart. And no creature is hidden from his sight, but all are naked and exposed to the eyes of him to whom we must give account. - Hebrews 4:12
When one reads or hears things of themself that only they know to be true, their ears (or eyes) tend to open a little wider and pay a little more attention to what the writer has to say. And fwiw, I think we are all inquisitive in that regard - wanting to know the truth, particularly of things we only have an inkling. Some go to the Tarot, mediums, soothsayers, the daily horoscope, their psychologist, or family, friends; some seek it through experiences, education, etc.
But those who have taken the time to actually sit down and read the bible, or have someone explain it to them as they read, discover truths about themselves, about others, about the world - seeds of truth that in time (and study) grow into a genuine faith.
...faith comes from hearing, and hearing through the word [of Christ]. - Romans 10:17
So I guess in answer to your question - "how do we know the bible is from God?" - faith. We trust what we've read and studied, having overlaid it with what we know to be true, tested it repeatedly (and always) for consistency, and having found no loopholes, conclude as we do.