I get the idea that Essence isn't supposed to be, in-universe, something quite so "by the numbers". For the sake of balance and playability, it works very clearly for the players, but the people in Shadowrun probably can't always tell what is the limit; if it were so clearly defined, stuff like cyberzombie "research" probably wouldn't be as much of a thing, as if they just knew "this is how much Essence someone has, end of story", it'd be pointless to try and go past that number (the only useful thing would be to try and make more efficient augmentation).
In Shadowrun Returns: Hong Kong they make it clear that people on the street (including the skilled street docs) don't know the exact concept of Essence; just a "rule of six", in that there's six potential augmentation zones (four limbs, head, torso) and stuffing extra in is bad. In 20 years, I'm sure they've learned more, though.
Keep in mind that the concept of Essence is a magical concept; there's people who believe it'd BS entirely because it comes from magical research over purely scientific research, as it's often called things like "holistic integrity" and so forth. As with anything magical, you can never boil it down to the exact specifics, because magic just doesn't work that way. But it's understood through a combination of medical science and magical research that augmentation damages your aura, and that an aura made too weak means a person will die.
However, just like a PC magician still rolls dice the same way as someone firing a gun does, just because it's more nuanced and vague in-world doesn't mean it should be for the characters. I would never hide a PC's Essence score from them (even if it were possible somehow); knowing it and being able to micro-manage that amount is a big part of the balance for mundane characters. A player should never have to actually gamble on whether or not they permanently lose their character just for progressing them as intended.
An Awakened person can get the out-of-game number on someone's Essence, but in-game it wouldn't be a literal number; I mean, even the basic concept of "what is astral perception like?" is something that differs for each magician and explicitly is incredibly difficult to describe to someone else. In-game, they'd be able to look at someone else and know "that person's aura is about as damaged as yours", rather than just comparing it directly to "equal, higher or lower than my own". They would be able to use vague terms to describe it to someone else "their aura is very thin", "I can see slight discoloration", "what remains of their aura is frail". And so forth.