We can know that the Book of Mormon is true because it was written so quickly!
www.lds.org: One of the most amazing facts about the Book of Mormon is that it took Joseph Smith only about sixty-five working days to translate a book that, in the current edition, is 531 pages long.
We can know that the Quran is true because it took so long to write!
www.thewaytotruth.org: The Quran was revealed in 23 years, yet there are not any contradictory points in it. It is inconceivable that any book written by a mortal being in 23 years, one which is a book of Divine truths, metaphysics, religious beliefs and worship, prayer, law and morality, a book fully describing the other life, a book of psychology, sociology, epistemology, and history, and a book containing scientific facts and the principles of a happy life, does not have any contradictory points. Whereas, the Quran openly declares that it has no contradictions at all and therefore is a Divine Book.