In the process of revealing himself to his brothers, Joseph arranged to have his divining cup hidden among their belongings:
And he commanded the steward of his house, saying, Fill the men's sacks with food, as much as they can carry, and put every man's money in his sack's mouth. And put my cup, the silver cup, in the sack's mouth of the youngest, and his corn money. And he did according to the word that Joseph had spoken. As soon as the morning was light, the men were sent away, they and their asses. And when they were gone out of the city, and not yet far off, Joseph said unto his steward, Up, follow after the men; and when thou dost overtake them, say unto them, Wherefore have ye rewarded evil for good? Is not this it in which my lord drinketh, and whereby indeed he divineth? ye have done evil in so doing. (Genesis 44:1-5)
To help you understand the significance of this event, The Dake Annotated Reference Bible offers the following explanation, on page 44 of the Old Testament:
This was to impress them that Joseph was an Egyptian. Divining cups were used among the Egyptians as well as other nations. They bore certain magical inscriptions. They were filled with pure water and in some cases melted wax was poured into the water and the will of the gods was interpreted by the shapes of the wax. Or, gold and silver and precious stones with magical characters engraved on them were thrown into the water. Then words of incantation were muttered, and some of the signs engraved on the stones, being reflected by the sun in the water, were interpreted as meaningful.