When I think of how I know, it all comes down to one thing: the spirit of God. It is thanks to the mercy of God allowing a portion of His spirit to be with us that we are able to discern light and truth. I feel so humble and grateful when I receive peace about a concern, an answer to prayers, or joy in simple things. Indeed, those are tender mercies from the Savior.
One of those mercies is my testimony of Joseph Smith. I remember having a testimony of His calling and mission when I was a teenager. It was just that simple. When my older brother got home from his mission however, he shared some things with me about the Prophet's character and virtues. He told me he read that Joseph's axe yard was always an example of order and he could do three times the amount of work in one day an ordinary man could do (it could mean he was just strong, but I also took that to mean he was a hard worker). He also told me of his extraordinary ability to connect with children, how he played with them and didn't shoo them away as many would. These little details prompted me to learn more about the prophet's history and read the Doctrine and Covenants. My study and prayer on these matters brought stronger witnesses of every step of the restoration of the gospel, including priesthood ordinances and temple covenants.
I know that Joseph had visions and saw angels. I know that he saw God the Father and His Son Jesus Christ who really spoke to him. I know the Book of Mormon is a true record of ancient civilizations that lived on this continent and knew and worshiped God intently. I know the Book of Mormon was written on gold plates and delivered to Joseph Smith and translated by the power of God. I also know he saw miracles in protecting those plates. He saw miracles during each step of restoring all keys, truths, and rites of the gospel of Jesus Christ to the Earth. God is all-loving and wants us to have everything that is good and pure. I know God lives, I know Christ lives and I know there is a true prophet living today through whom God speaks to us all and guides us in the right direction.