Let's see. God is a god of truth, cannot lie, He loves his children, will not lead them astray, He's perfect in every way, and is unchanging.
The Godhead included Heavenly Father, Jesus Christ and the Holy Ghost. All are "God", perfectly united in their love, knowledge, power, motivation, purposes.
God, in this case the Holy Ghost, has told me clearly that the Church is true, that Jesus Christ is our Savior, that Joseph smith is the Prophet of God through whom Jesus Christ restored His gospel, His Church and His Priesthood, and brought forth the Book of Mormon for our benefit, and that Thomas S. Monson is God's Prophet living on the Earth today.
So, since God told me these things, then yes, I do "know" them as true fact, incontrovertable, beyond any doubt or question.