Apparently, after some excavations in Mesopotamia, inscriptions of predictions (models) were discovered. Based on how the liver of a ruler/king or a noble was shaped, predictions were made by priests. They would associate the shape of the liver to a pre-made model which was to foresee the future.
In the course of Parrot’s excavations at Mari in 1935-36, 32 inscribed clay livers were discovered among the tablets found in Room 108 of the palace. This piece comes from that group, which offers the earliest direct evidence of hepatoscopy (the examination of the liver of a sacrificed sheep), one of the branches of ancient divination. A practice proper to Mesopotamia, the art of divination produced a considerable body of specialized literature: haruspicy, the observation of animal entrails, and astrology, a later development, were the two most important forms. Dreams and birth abnormalities were also interpreted to yield predictions. Hepatoscopy developed in the early second millennium BC and became increasingly important. Although it is attested in the pre-Sargonic period (third millennium BC), the Mari livers are nonetheless the oldest known representations of omens.
Read all about it at: http://www.louvre.fr/en/oeuvre-notices/model-liver-divination