Throughout history the military has always defined ranks, it would only logical to assume that even the common soldier would be able to tell who the officers and commanders would be, especially because in the heat of battle, when all is in disarray one cannot go around shouting, 'hey who is leading us?'
They commanding officers would usually display differences in outfit or symbolism to show rank.

However, we might assume that Arn simply isn't familiar enough with the knights and their officers, since they might be kept and trained separated, only seeing each other on the battlefield.