Centurions

[A_Volpe]

Some of the higher ranking Romans in T's C are recognisable.

With apologies, slightly twisting your words here

It may be possible that soldiers were able to recognize their officers by their face, and probably by their individual kits (ie things like torques, dona, perhaps even armor and weapons)
And relied less on what their helmets looked like - Seeing as it's possible that a majority of the helmets in use are among the same kind of shape and style.

As mentioned in another post, Fashion Dictates. I realize it's not a great comparison, but looking at the 17th century, being where the [Dutch] are copying 'everything' Roman and Macedonian for their armies - they'd want to "stand out" on the field by what they are wearing. I do not recall seeing a reference about knowing who your captain was solely by his helmet (or hat) - but by his entire outfit, inasmuch as what he carried (ie: Partisan, Halberd et cetera, Sash)

To take a real stretch, looking at the Fayum portraits, there is that one that a number of people strongly argue is a portrait of a Roman Centurion - I believe in addition to his clothing, he is wearing a gold leaf crown / Corona (which is what I believe people call out as to why he's called a Centurion in the first place)

Also, considering a Centurion is not going to wander off from his "post" at his position in the Century when deployed, the Soldiers I'm sure would be confident to know where to look and where to expect their commander to be at any one time. When in camp, I'm pretty sure the soldiers would be able to recognize their officer's face after a short amount of time (easily done when he's glaring at you)

This fashion and position to signify one being an officer remains in effect will into the modern age. Officers in the American Civil War typically wore their own custom made clothing, and of styles that were different than the regular soldiers. Zouaves are a good example of this, both in French/European and US aspects.

I don't know how it works in the modern military, but, can a soldier easily tell their commander out of a crowd? Distinctive voice and mannerism?
We even see this is cartoons - notably with "Beetle Bailey" - it is very easy to tell Sgt. Snorkel from Beetle or the other soldiers; Lt. Fuzz from Gen. Halftrack, et cetera....You always know it's going to be Drill Sgt. Hartman / R. Lee Ermey because of his voice and his mannerisms, his cover/lid only adds to his "look". You hear Ermey's voice in a Radio ad, I am sure you will conjure up an image of him with his cover on his head...Screaming at you...

Also, The centurion "Hand me another" wasn't given that name because of what his helmet looked like.