Probably one of those techniques where they observe how something burns.

A LITTLE HISTORY

In ancient Rome, there was an official College of Augurs whose appointed duty and whole job was to observe carefully any and all omens arising from natural causes, as well as those purposely-induced by artificial means (such as alectryomancy, oil divination, grain or sand divination, etc.), and to interpret these omens on behalf of the Roman government, who allowed their decisions to be influenced by the augur’s interpretations of the omens.

There is an important distinction to make here. The Augurs were not the same as Oracles, like those at Delphi, Argos, and Dodona. Augurs observed omens; Oracles channeled messages while in a trance state.

Roman Augurs divided omens into two main classes:

  1. Omens received in answer to a definite question. Alectryomancy, oil divination, and grain/sand divination are all examples of this first sort of augury.
  2. Omens unsought and observed by chance. Bird divination, animal divination, and aeromancy (atmospheric divination) are all examples of this second sort of augury.

Roman Augurs sought omens mainly using the following five methods:

  1. Aeromancy;  those seen in the heavens, such as thunder, lightning, hail, falling stars (meteors), comets and the like.
  2. Ornithomancy; the flight and sounds of birds.
  3. Animal divination; the cries of animals and general observations obtained from their behavior.
  4. Alectryomancy; the ways in which birds picked up grain spread out for them to eat.
  5. Any accidental or unusual happenings.

The Roman College of Augurs appear to have employed the first type of augury on a more-or-less regular schedule. At midnight, the appointed Augur went to a specific hill where he had an uninterrupted view of the city and the surrounding countryside. First he offered up a sacrifice to the Gods, beseeching Them to send him true omens in the heavens. Seating himself in a tent and facing south, he then patiently awaited as answer from the Gods. An example of the sort of omen he might be looking for is, if lightning flickered in the east on his left side, it was regarded as a good omen. If the lightning flickered in the west on his right side, it was regarded as a bad omen.

For the record, the ancient Greeks, who also used augury, attached greater significance to omens of the fifth type listed above. And for them, the order was reversed; omens observed on the right side were lucky, omens observed on the left side were regarded as unlucky. Which just goes to show omens are somewhat dependent on generally-accepted norms of the society in which they are seen, because omens, by their very nature, are something outside of the norm—an animal acting strangely, the wind blowing something toward your feet, bizarre-looking clouds.

History is riddled with incidents where omens received at critical moments influenced the course of subsequent events significantly. Two armies lined up to do battle, when a hare runs straight between the two sides, thus stopping the battle. A future Roman emperor, napping outside, wakes up to see an eagle circling over him, and knows it predicts future leadership for him. Another Roman Emperor, facing a battle where he and his army are outnumbered and likely to lose, claims to have seen an unusual atmospheric phenomenon which inspired him to win the battle, change religions, and in so-doing change the future course of western civilization to a huge degree.

In time, the Roman College of Augurs would go out of existence, but some of their standards of observation passed into folklore and have survived down to the present day.