At the end of the Book of Mormon, Moroni invites the reader to pray and ask God if the book is true. Moroni 10:3-5 reads:
Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts.
And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
And by the power of the Holy Ghost ye may know the truth of all things.
Since the restoration of the church, millions of Latter-day Saints have claimed to pray and feel a witness of the truth by the power of the Holy Ghost. Why think they are all mistaken?