I cannot say that I have a specific moment when I have known that Joseph Smith was a prophet, but a lifetime of answered prayers has built that testimony for me. My testimony is like a piece of coal, and with each prayer, with each blessing, with each trial even, and with each joy I experience through the things that the prophet Joseph Smith restored the coal hardens toward a diamond. I do remember one specific prayer I had. It was an earnest prayer, and I recieved an answer. It was particularly poignant, but not earthshaking. Indeed, I continue to have moments everyday. It is built on reasoning and the testimony born to me by the Holy Ghost. It resides in my mind and in my heart. I believe Joseph Smith was a prophet, and I believe all that he revealed, including the Book of Mormon.