Walton et al. provide a scholarly overview of ancient divination.
John H. Walton, Victor H. Matthews & Mark W. Chavalas, The IVP Bible Background Commentary: Old Testament (Downers Grove, IL: InterVarsity Press, 2000), 74
divination cup. The cup that Joseph plants in Benjamin's sack is identified as being used for divination. Just as tea leaves are read today, the ancients read omens by means of liquid in cups. One mechanism involved the pouring of oil onto water to see what shapes it would take (called lecanomancy). More popular methods of divination used everyday occurrences, configurations of the entrails of sacrificed animals or the movements of the heavenly bodies. Lecanomancy was used in the time of Joseph, as is attested by several Old Babylonian omen texts concerned with the various possible configurations of the oil and their interpretations. Another technique, hydromancy, made its observations from the reflections in the water itself. Not enough is known about Egyptian divination techniques to offer more specific information, but in there early periods typically only people of status had access to divination procedures.