When surcoats began being phased out of use towards the end of the medieval period, how would heraldry be displayed when men were fully armored? Were there ways to tell who was who without knights having their crests displayed on their armor?

(pictured above, a crusader’s surcoat)

During the “surcoatless period,” which was a real phase in the history of European armor (1410-1430), people still used heraldry, they just didn’t use the surcoat because it was seen as unfashionable compared to wearing your armor “all white” (i.e, bare steel) and because it restricted movement. 

Instead, they would carry their heraldry just on banners and lance pinions, or painting it onto the armor itself, as we see here:

However, between 1430-1500, they used tabards or jupons instead, which were shorter (usually just to the waist) and either had shorter or no sleeves at all, so they wouldn’t impede movement, but they would still do the basic job of identifying who was on which side and who might be worth a ransom.