Lack of uniforms

[konny]

shield emblems...

Size, shape and signes of/on shields would have been the most common and best way to distinguish "us'" from "them". Everything else, save for the helmet, is hidden by the shield anyways. The most popular examples are from the Middle Ages with all those indiviual heraldic symbols painted on shields (even though it must have been a pain to learn these by heart for every knight of your side and recall it in the heat of battle).

Examples from the Ancient times would be the Greeks useing regional signs on their shield, or Constantine the Great useing the Christ symbol in the battle of the Milvian Bridge. I would assume that signs like that were also in use in former civil war battles or in battles in which soldiers with the same equippement (and the same shields) clashed.

Other methods of distinguishing became important and a habbit when the soldiers stopped useing shields in the early Modern Age. Uniforms were used widespread sinze c. 1700, but even these did not always help because only blue and white/grey were used, with very few expections (eg. red for British and green for the Russians). For example, at Waterloo Dutch and Prussian soldiers often fired at each other because both were wearing blue like the French.