how would medieval fighters/warriors/military recognize friend from foe?

vonstroheims_monocle

A contemporary described Edward I's army marching to Scotland in 1300: "There were many rich caparisons embroidered on silks and satins; many a beautiful pennon fixed to a lance, many a banner displayed." (quoted in Michael Prestwitch, "The English Medieval Army to 1485" in The Oxford History of the British Army) This description, though cropping the less glamorous foot soldiers from the picture of chivalric grace, gives some idea of the picturesque appearance of the chivalry of England, and indeed, of much of Europe, in the High Middle Ages.

The earliest means of recognition in the Middle Ages was through designs on the shield. Since at least c.800 BC, the shield was painted with unique designs in order to distinguish a fighter in the field. This practice was in widespread use amongst the Franks, Saxons, and Norse.

Even prior to the widespread adoption of all-enclosing Great Helms which totally obscured a Knight's face, recognition was difficult on the battlefield. At the battle of Hastings, news spread amongst the Norman ranks that William had been fatally injured. In order to be recognized, William lifted his helmet so he could be seen by his men. At this stage, the helmet would likely be a nasal helm, as depicted in the Bayeaux tapestry. Presumably, the nasal guard was wide enough to make recognition difficult, though it should be noted this anecdote was written substantially later.

One of the most apparent and well-known means of recognition on the Medieval battlefield was heraldry. Heraldry differed from earlier decoration in that it was a symbolic insignia which was inherited from generation to generation. In use since the mid 12th century, heraldry was at first restricted to the shield, with the surcoat, and horse trapper being of different colors. Helmet crests were in use since the 1190's, as the great seal of Richard I shows. However, prior to the introduction of more elaborate crests, the helmet was painted in a variety of colors. The beautiful Maciejowski Bible depicts Knights c 1250 bearing heraldic devices on their shields, with helmets in the color of their arms, though their surcoats and horse trappers are of unrelated colors. Compare this with a miniature from The Grandres Chroniques de France, c. 1332-1350, of Knights in combat. Though military equipment has changed but little (most notably, the great helm has assumed a 'sugarloaf' shape and leg defenses are in use), the heraldic designs are much more apparent on throughout the knights' costumes.

Shields, crests, surcoats, ailettes, lance pennons and horse trappers bore a knight's arms, distinguishing him on the battlefield. Contemporary illustrations, like this one of Geoffery Lutrell c. 1340 and these figures of Hartmann von Aue and Ulrich Von Liechtenstein from the Codex Manesse c. 1300, give some idea of what the Knight of the last quarter of 13th and the first half of the 14th appeared when emblazoned with heraldic devices.

In order to interpret this dizzying array of symbols, military commanders employed heralds within their army. The role of the herald was, initially, to introduce a knight at tournament by calling out his deeds. They soon took to organizing tournaments, and by necessity were forced to become experts in heraldry. There use in actual war (though the distinction between war and tournament in the 12th century was less apparent than might be assumed) was very apparent. The herald, with his wide knowledge of the arms of knight and nobles, could recognize those on either side of the battlefield. Heralds became appointed officials and confidants, and were employed by Kings and Lords throughout the 14th century.

Use of the surcoat declined during the 14th century, a transitional period for arms and armor. In its stead, Knights wore the jupon, a tighter fitting garment often of cured leather. Only England was the jupon widely emblazoned with a heraldic device, as depicted on the effigy of Edward, the Black Prince. On the continent, it was likely to be unmarked. Use of the shield declined as well, till it was all but abandoned, save for use in tournament, by the 15th century. The rise of plate armor meant the shield was obsolete, and it was fashionable to wear armor 'alwite', or 'all white', without decoration or covering. The knight's arms were restricted to his banner and lance pennon. The surcoat, now in a loose fitting form known as a tabard, was worn but rarely.

The 14th and 15th centuries saw the widespread adoption of liveries, the forerunners to military uniforms. During his campaigns in France, Henry V ordered every man wear a garment bearing the cross of St. George. In England, under the system of Livery and Maintenance, soldiers entered a lord's service, agreeing to wear his livery, distinctive colors, and badge. Sometimes, livery colors corresponded to the colors on the Lord's arms, though this was not always the case. The badge, too, was not always a heraldic device. Though the conclusion of the War of the Roses extinguished the Livery and Maintenance system, liveries and badges continued to be worn by bodies of troops raised during the Tudor period, and uniforms in livery colors were worn until as late as the 17th century.

Sources:

Barber, Richard. The Reign of Chivalry (David and Charles, 1980)
Edge, David and Miles Paddock. Arms and Armour of the Medieval Knight (Bison Books, 1988)
Prestwitch, Michael "The English Medieval Army to 1485" in The Oxford History of the British Army, ed. David Chandler and Ian Beckett (Oxford University Press, 1994)
Wilkinson, Frederick. Battle Dress (Guinness Signatures, 1970)
Wise, Terrence. Medieval Heraldry (Osprey, 1980)