There has been controversy about whether or not Mormons are true believers in Christ and if they are saved. Joseph Smith was encountered the same way Mouhamad was; by and Angel claiming to bear witness of GOD. It is very clear what the bible says about these type of preachings and what you can point to when you encounter these faiths:
2 Corinthians 11: 13-15
A great fact is that these books were written 1 generation from Jesus and sealed for eternity by the Apostles of Jesus Christ himself. Islam was started by an angel of light 650 years later and Mormonism even much longer after that .