It is, however, a trope in lots of media that a trained warrior can size up opponents instantly. Some can go even farther, an predict how a fight will go (the "Awesome by Analysis" trope in action- some examples include Jack Reacher or the movie version of Robert McCall). In Asian media, opponents can envision entire battles before a single sword is swung (there's a nice example of this in Jet Li's "Hero").
Is it realistic? I don't really care. It's entertaining! And so what if it's fantastic? Isn't D&D a fantasy game? What's wrong with having fantastic elements in your fantasy game? Too often, there seems to be this line in the sand about what fantasy is acceptable for D&D and what isn't that just boggles the mind.
I mean think about it, you want to say that characters and creatures in the game don't know what an opponent's AC is...but at how many tables does a character or creature know what the result of an attack roll is so that they can use a defensive ability like Shield? Ditto for knowing what a damage roll result is so you can decide if this is the moment you should use an ability that reduces incoming damage.
And even as I type that, I'm sure I'm going to get comments such as "I ban Shield", "I never tell my players how well they were hit", and "I never tell players how much damage they take, I merely describe the hits", lol.