Angel, Spirit, Devil.

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.
- Doctrine & Covenants: Section 129, verses 4-9