The inspection came immediately after the killing and before anything else could proceed. The animal was opened and the exta lifted out: the liver above all, with the gall bladder and the fleshy projection on it, the lungs, the heart and the membrane around the organs. The first reading was pass or fail. If the organs were sound and correctly formed the offering had been accepted and the business in hand could go forward; if they were not, the rite had failed, and another victim was brought, and if necessary another, until acceptance was obtained. Roman writers record occasions when a magistrate went through many animals in a morning.
Only after acceptance did the detailed reading begin, and it was the liver that carried it. The organ was treated as a map. Its surface was divided into regions, and each region was assigned to a divine power, so that a mark, a swelling, a discoloration or a malformation in a given area was a communication from a particular quarter. A part of the liver was read as favourable to the consultant and a part as favourable to the adversary, so the same feature could be good or bad depending where it fell.
That the reading belonged inside the sacrifice matters. This was not an oracle consulted separately from worship, and the practitioner was not asking a question so much as checking whether the transaction had gone through. Extispicy of this kind is much older and much wider than Italy, and is documented in detail in Mesopotamia centuries earlier, which is one of the standing questions about where the Etruscan version came from.