Instructions given by Joseph Smith the Prophet, at Nauvoo, Illinois, 9 February 1843, making known three grand keys by which the correct nature of ministering angels and spirits may be distinguished (see History of the Church, 5:267).
How to identify a demon
Joseph Smith
Doctrine and Covenants Section 129
1–3, There are both resurrected and spirit bodies in heaven; 4–9, Keys are given whereby messengers from beyond the veil may be identified.
1 There are two kinds of beings in heaven, namely: Angels, who are resurrected personages, having bodies of flesh and bones;
2 For instance, Jesus said: Handle me and see, for a spirit hath not flesh and bones, as ye see me have.
3 Secondly: the spirits of just men made perfect, they who are not resurrected, but inherit the same glory.
4 When a messenger comes saying he has a message from God, offer him your hand and request him to shake hands with you.
5 If he be an angel he will do so, and you will feel his hand.
6 If he be the spirit of a just man made perfect he will come in his glory; for that is the only way he can appear;
7 Ask him to shake hands with you, but he will not move, because it is contrary to the order of heaven for a just man to deceive; but he will still deliver his message.
8 If it be the devil as an angel of light, when you ask him to shake hands he will offer you his hand, and you will not feel anything; you may therefore detect him.
9 These are three grand keys whereby you may know whether any administration is from God.
Joseph Smith