Samuel Smith “knew his brother Joseph had the plates…and he had handled them and seen the engravings thereon”
In the spring of 1832:
Elder [Samuel] Smith read the 29th chapter of Isaiah at the first meeting and delineated the circumstances of the coming forth of the Book of Mormon, of which he said he was a witness. He knew his brother Joseph had the plates, for the prophet had shown them to him, and he had handled them and seen the engravings theron. His speech was more like a narrative than a sermon.
Phineas Young (brother to Brigham Young) met Samuel on his first mission, and was given a Book of Mormon:
“Ah,” said I, “You are one of the witnesses.”
“Yes,” said he, “I know the book to be a revelation from God, translated by the gift and power of the Holy Ghost, and that my brother Joseph Smith, Jr., is a Prophet, Seer and Revelator.”