This description would help the brothers see that the ruler knew things that other people could not know.
Hydromancy (trying to learn hidden things by pouring water into oil) and oenomancy (trying to learn hidden things by pouring wine into other liquids) were forms of divination (seeking knowledge through supernatural means) used in the ancient Near East. A special cup was often used for these practices.
Joseph was still pretending (see 42:7; compare 30:27; see also Leviticus 19:26; Numbers 23:23; Deuteronomy 18:10–11). He knew that only God reveals the future (see Genesis 37:5–9; 40:8; 41:16).