How do I know? Because I have asked and God has answered.
What Moroni brought to pass by leading Joseph to the plates has been nothing but good. Would Satan disguise himself as an angel, to provide a book which leads men to Christ?? I think not. He cannot. It is impossible for a good tree to bring forth bad fruit and a bad tree to bring forth good fruit.
And by their "fruits" ye shall know them. The fruit of the gospel and of the LDS church is: Love, charity, Christ-like attributes, honesty, integrity, high morality, love for God and His Son. You think Satan would want those things to come about? Um no.
Jesus NEVER claimed he was God. Ever. He claimed to be Christ and the SON of God. Which He is. He taught to worship the Father and to give all glory to God.
The angel Mornoi never told joseph other religions were corrupt. Christ and God did. They told Joseph all other religions' CREEDS were an abomination. That they had parts that were correct, but parts that were not. That is why there was a need for the restoration of the gospel.
It doesn't matter how much "evidence" is out there, for OR against, what happened. A person can NEVER fully know the gospel is true until they ask God. He has promised He WILL answer.
One more thing...as someone else pointed out - other beings appeared to Joseph and others. Oliver Cowdery was present when Peter, James and John came to confer to Joseph and Oliver the Melchezidek Priesthood. Also John the Baptist.
http://scriptures.lds.org/en/dc/110
The link is the vision given to Joseph and Oliver Cowdery at the Kirtland Temple when Christ appears and accepts the temple as His house. Also Moses and Elias appear, as well as Elijah as promised by Malachi.
7 of 9 - So there is no possible way an angel from God would appear as an angel of light, hu? It had to have been satan?? WHY would satan appear and speak of Christ? HE WOULDN'T.