There are several approaches to this:

  1. Divining by ashes as the fire is still burning. If the ashes rising from the fire disperse quickly, that is interpreted as a positive sign. If the ashes rising from the fire seem mostly to hang in the air, that is interpreted as a negative sign. This is more of a general atmospheric influences way of reading the ash.
  2. Spodomancy as a yes/no method. When a positive or negative omen is sought, cooled ashes are scattered evenly on a hearth, floor, or ground, then the inquirer takes up a pointed instrument and makes a random number of lines in the ashes. An even number of lines is a positive omen. An odd number of lines is a negative omen.
  3. The Romany Method. Another way of drawing a positive or negative omen from ashes is the Romany method; gather up the ashes from a fire and cast them on the floor. If the ashes land in one or more uneven piles on the floor, it’s taken to be a bad omen. If the ashes land in a smooth, uniform layer, then it’s taken to be a good omen. If you have a rather intermediate result, where the ashes are neither a smooth, uniform layer nor exactly in piles, then you can assume the energy is mixed; some of it good, some of it bad.
  4. The Question-and-Answer method. This comes from nineteenth century Germany. Ashes are scattered thickly in some place exposed to air, and the person wanting an answer to a question writes their question into the ash with a fingertip. The ashes are left there for the night. The following morning, the ashes are inspected and any letters which remain legible are taken as an oracle of some kind. This morning-after analysis is to be treated one of three ways: (1) either the still-legible letters, taken in order, form a word, (2) the diviner lexigrams a word from the left-over letters, or (3) the diviner determines if the remaining letters form an acronym. Should other characters appear in the ashes which weren’t written the night before, tradition dictates the additions be ascribed to the devil and therefore disregarded. The danger with the question-and-answer approach to ash divination is others, not just the devil, may find it amusing to interfere with the ashes and mess-up the answer you get. 
  5. Cineromancy. In cases where a ritual fire is burnt, as in the case with a sacrifice or otherwise, the remains of the fire is allowed to cool down, then the unburnt wood or other unburnt fuel is carefully removed. The ashes are then studied, observing any ridges, mounds, valleys, or other imperfections in the surface of the ash. Special attention should be paid to where these elements intersected or where they dead-ended with the unburnt parts. Were the ashes all going smoothly until they suddenly dead-ended in some unburnt wood, or was there a ridge of ash, then some unburnt wood? Do the ashes form a picture? Are they pointing to anything in particular? This is an approach ash divination where your intuition plays a huge part.