I can't answer for Western troops at least. What I can tell you about is a bit how the Byzantines did it in the year 610ish.

The source of all of this information is from the Strategikon, a military manual written by a ghost writer (most likely) for the Emperor Maurice. If you want to get into the really nitty gritty details of the early medieval world's best trained and most organized army, this is what you should read.

Familiarity of the people who surround you is one reason. The Byzantines were highly regimented, and one of the things they did to highlight this is that their camps were set up the exact same way every single time they set up camp. This was originally designed to help repel night attacks, as part of the confusion of night attacks was that it was dark and you couldn't know where you were going, but the Roman soldiers knew their camp was always the same. This was done with your unit, so it meant that every single night, Byzantine troops would sleep around and be around their fellow soldiers. This also meant that you would know what your comrades looked like, helping ensure that you didn't stab each other in battle! It's the more minor of the two reasons, but I think it's worth mentioning.

The other reason was because of the highly intricate banner system. Each unit, still a centuria and still commanded by a centurion, had a standard that was similar to the stand of the old legions. There were so many that supposedly Heraclius found if I remember correctly about 200 captured standards in a palace of Khosrau II. They were each unique, so the men knew what standard they were following and as such knew where they were and could also see where the other banners were. Before battle as well, there were a series of traditional commands barked out by the commander, one of which was "do not go past your standard." So they soldiers knew that they were to remain with their standard, and the strict organization of the Roman formations meant that they would pretty much knew where friend and foe were at all times.

Hope this can kind of answer :)