Banners, personal recognition, style of dress, surcoats/tabards/aketons, relative position on the field.

It must be noted that the transition between 'feudal levies' and 'standing armies' was not a smooth or linear one. One can examine the English garrison in Calais as a long-term force in the narrow sense, yet these were still essentially mercenaries on contract; owing allegiance to the commander of Calais. During the Hundred Years War, large proportions of the English armies were hired by the system of 'indenture'; essentially, mercenary contracts. Thereby, they were often personally familiar with those around them, but also were given 'liveries', the precursors to military uniforms.

From this, we can ascertain that there was some changes from the era of the late 13th century, but that the majority of /u/vonstroheims_monocle 's answer here is still valid. As he notes in his answer 'liveries' became very common during the Hundred Years War, moving from the retinues of Henry of Grosmont and Edward III into those of the wider nobility. This essentially meant that you could recognise a man's allegiance by his dress, by the badge on his jupon, and the myriad of personal embellishments that the nobility acquired for themselves.