Why Ask?
LDS Canonical Book: Doctrines & Covenants 1:37 - “Search these commandments for they are true and faithful, and the prophecies and promises which are in them shall be fulfilled.”.
Bible: 1John 4:1 NASB - “Beloved, do not believe every spirit, but test the spirits to see whether they are from God because many false prophets have gone into the world”.
Joseph Fielding Smith, the tenth president of the LDS Church, maintained that: “Mormonism, as it is called, must stand or fall on the story of Joseph Smith. He was either a prophet of God, divinely called, properly appointed and commissioned, or he was one of biggest frauds this world has ever seen. There is not middle ground. If Joseph Smith was a deceiver, who willfully attempted to mislead the people, then he should be exposed; his claims should be refuted, and his doctrine shown to be false … I maintain that Joseph Smith was all that he claimed to be”. (Doctrines of Salvation, 1959, Vol. 1, pp. 188-89).
How Determine?
Deut. 18:22 NASB - “When a prophet speaks in the name of the LORD, if the thing does not come about or come true , that is the thing which the LORD has not spoken. The prophet has spoken it presumptuously ; you shall not be afraid of him”.
If one prophecy fails, the person is NOT a prophet.