The most common method being to scatter grains, for example on an alphabet, and seeing which ones the big guy picks up. But there are other ways of observing meaning in the stochasticity of cocks, including interpreting scratch-marks left on the ground or the innards of a sacrifice. Also spelled alectromancy and alectormancy. From Greek, from alektōr, rooster + -manteia, divination -- and that first word is related to the name Alexander, both being derived from a root meaning to ward or drive off.
This painting by Konstantin Makovsky shows young women using alectryomancy to fortell marriage prospects: