If Mormonism were true…
1. … Joseph Smith would have been able to consistently, accurately, remember his visit from two separate supernatural beings, God the Father and Jesus the Son. We now know that is not the case (for more info, see here, here, and here).
2. …the LDS “truth” that there are at least two gods, our Heavenly (spirit) Father, whose name is Elohim, and Jesus (Elohim’s son), whose spirit-name is Jehovah, would have been consistent since Mormonism’s beginnings. Instead, the Book of Mormon, Joseph Smith’s first work, preaches that there is only one (modal) God (see Alma 11:26-31, 2 Nephi 31:21, Mosiah 15:1-5 for example). Joseph Smith originally taught that Jesus’ father’s name was Jehovah, and Brigham Young, Mormonism’s second prophet, taught that Heavenly Father was actually Adam, the first man on earth. Joseph also originally taught that of the three members of the “Godhead,” only Jesus had a body. That of course is no longer Mormon belief.
3. …the Book of Mormon, a book that is supposed to contain the “fulness of the gospel,” would teach on the plurality of gods, man’s potential for godhood, eternal marriage in Mormon temples, baptism for the dead, three degrees of heaven, and the other beliefs that separate Mormonism from orthodox Christianity. Not only does the Book of Mormon not teach these things, it and the other LDS scriptures frequently contradict current Mormon truths, and each other.
4. …there would be evidence of a large battle on or around the Hill Cumorah in New York, and other archeological evidence to support the notion of Book of Mormon life on this continent. Instead, LDS apologists are still struggling to locate and identify possible Book of Mormon geography sites (see also here and here).
5. …the Book of Mormon would not contain Greek and French words like “adieu” and “Jesus” and “Alpha” and “Omega.” It would not speak of things that had not yet been invented. It would not contain quotes from the KJV Bible, including KJV mistakes. It would not abuse the phrase “it came to pass” in all of its books but two. It would not contain country-boy vernacular. But it does.
6. …when portions of the papyri used to create the Book of Abraham were recovered and translated, the text would be very similar to what Joseph had written. Instead, just as you would expect if Joseph Smith had bought merely a couple of random mummies that had been found in an ordinary catacomb from a man with many mummies and scraps to sell, the papyri has been discovered to be common Egyptian funerary documents.