It would have been equally as difficult for us to tell by what spirit the prophets prophesied, or by what power the apostles spoke, and worked miracles. Who could have told whether the power of Simon, the sorcerer was of God, or of the devil?…if we go among the Pagans they have their spirits, the Mahomedans, the Jews, the Christians, the Indians; all have their spirits, all have a supernatural agency; and all contend that their spirits are of God…A man must have the discerning of spirits as we before stated to understand these things, and how is he to obtain this gift if there are no gifts of the spirit? And how can these gifts be obtained without revelation?” – Joseph Smith
1 Samuel 28:11-13; “Then said the woman, Whom shall I bring up unto thee? And he said, Bring me up Samuel. 12 And when the woman saw Samuel, she cried with a loud voice: and the woman spake to Saul, saying, Why hast thou deceived me? for thou art Saul. 13 And the king said unto her, Be not afraid: for what sawest thou? And the woman said unto Saul, I saw gods ascending out of the earth.”
Acts 8:9-11; “But there was a certain man, called Simon, which beforetime in the same city used sorcery, and bewitched the people of Samaria, giving out that himself was some great one: 10 To whom they all gave heed, from the least to the greatest, saying, This man is the great power of God. 11 And to him they had regard, because that of long time he had bewitched them with sorceries.”
We test the spirits with the word of God. We use the tools God gave to us through His word to determine if what someone says is true and in addition to that we have the indwelling of the Holy Spirit – Hebrews 4:12.
The Jews knew Endor was a witch and they knew better than to go seeking her services because God had already told them to stay away from people like that. Endor didn’t really bring up the spirit of Samuel – the spirit she brought up out of the fire was a pagan god. God gave specific laws to stay away from anyone with familiar spirits – i.e. false gods in Leviticus 19:31.
As for Simon the sorcerer in the book of Acts once again we see how God intervened with His Spirit by bringing the apostles in to teach people his teachings were false. Smith can say what he wants about needing one special person to reveal the truth to people, but the job has already been filled by the Holy Spirit! Is Joseph implying he’s God?