How does one know that any book is authentic?
There is no absolute proof.
One looks for consistency over a long time and corroboration by outside sources of the time.
The Dead Sea scrolls indicate the Church’s scrupulous protection of the text from change/editing/misprints for at least 2000 years.
The bible is most likely the most researched, debated, analyzed, and interpreted text in the history of man. I think we should feel comfortable that it is authenticly a 2000-3000 year old text that has not been changed.
As others have said, if one accepts the Bible’s age and that its text is authentic and original (although not necessarily accurate in its intent), then analysis of its ‘‘prophecies’’ add comfort to one’s Faith in its source.
There is no proof it is God’s word, just as there is no Aristotlean proof that there is a God. But IMO God doesn’t want there to be proof of either, so as to accomplish His goals.