In Rome this was constitutional machinery. The augurs were a college of public priests, membership was a political prize, and no assembly could meet and no army could move without the auspices being taken. The question put was not what will happen but whether the gods approve of this action now, and an unfavourable answer stopped the business rather than predicting its outcome.
Two techniques are frequently confused. Augury proper concerns birds: their species, their direction of flight, their cries, observed within a marked region of the sky. Haruspicy reads the entrails of a sacrificed animal, above all the liver, and Rome took it from Etruria and continued to regard its practitioners as foreign specialists.
Because the procedure could stop public business, it was a political instrument, and Roman writers say so openly. A magistrate who declared that he had observed unfavourable signs could block a rival's legislation, and the abuse was notorious enough to be legislated about.
The story every Roman knew concerns a commander at sea whose sacred chickens refused to eat before battle, an unmistakably bad sign, and who threw them overboard with the remark that they could drink instead. He lost the battle and was prosecuted on his return. The anecdote is told as a warning about impiety and works equally well as a warning about morale.
Cicero, himself an augur, wrote a dialogue setting out the case for divination and then dismantling it.