How did knights identify themselves id their armor was so similar?

Heraldry came into play- designs such as various forms of crosses, ( from Crusades) and other insignia usually worn on the Breastplate- painted or etched on, or worn on someting fabric resembling a T-shirt worn over the armor. Thus national insignia ( banners were also used) rank insignia ( chevrons) and so on came into play. Usually Knight"s arms were based on family crests- naturally both religious and military designs were common, often intertwined. The official arms of Joan of Arc is basically blue and white, a white shield background. a bare sWord is shown pointing straight up and flanked by two fleurs de Lis ( signifying France, as does Blue and white color scheme) There is also what is either a crown or a ring at the apex of the sWord, if a crown, fidelity to King Charles Vii of France, if a ring, maybe target-proficiency is the idea as they used Brass rings for cavalry ( knights indeed) target practice with sWords or lances) int he middle ages. oddly for some reason the Heart symbol familiar to all on Valentine"s day, was not commonly used in heyday of heraldry, maybe because it was not known that the blood was circulated until l628- well after the days of knights in armor. There were some families ( Brown, Douglas) who had heart symbols on their family crests but not many. Animal designs , not confined to horses, were common.