Did some of them just leave the field?
I direct this at more complicated armies. Not at Crusaders vs Rebels. I mean clearly you could tell a crusader and a peasant apart. How did armies with generic armor tell each other apart from another army while in the midst of battle?
Also, how did you know a battle was won? Did people kind of just look around and say "Well crap, everyone is dead" and leave?
Edit: Also, I am aiming this question more in the early periods. Not in the 1600/1700s.
hi.. re question 1, you'll find some info in the FAQ*. There's one thread on the middle ages, but you may find interesting info in the others too
Identifying sides in battle, and avoidance/frequency of "friendly fire"
*see the link on the sidebar or the wiki tab
For a start, a knight would have his Heraldry or Livery which would be boldy displayed on his surcoat and/or shield. This would indicate his family, statues, and identify him on the field. Heraldry is incredible complex, but would go some way to point out a knight in the midst of battle.
In regards to battles; Most battles in the Medieval period would have been seiges, because a pitched battle was a risky affair which had to be organised by both parties. It's not often two forces on the march would just happen upon each other and form ranks.
Most casualties occured when one side routed, and was cut down by the chasing enemy. The melee would ebb and flow, with lines joining and breaking until one side runs or both withdraw.