“A man also or woman that hath a familiar spirit… shall surely be put to death...”—Leviticus 20:27

How do you know a “false spirit” didn’t answer your prayer when you prayed about the Book of Mormon?
“The Church of Jesus Christ of Latter-day Saints has also had its false spirits… it is not to be wondered at if there should be found among us false spirits.” —Teachings of the Prophet Joseph Smith, p. 213

“Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.”—1 John 4:1

Can Our Hearts Be Trusted?
“The heart is deceitful above all things, and desperately wicked: who can know it?” —Jeremiah 17:9

“When a man enquires of the Lord concerning a matter, if he is deceived by his own carnal desires, and is in error, he will receive an answer according to his erring heart, but it will not be a revelation from the Lord.” —David Whitmer, An Address to All Believers in Christ, p. 31

For additional information see:

Can Satan give Revelation?
Did Joseph Smith see God the Father?
What is Satan's Lie?
Is the Book of Mormon "the most correct book"?
Is the Book of Mormon compatible with the Bible and LDS Doctrine?