Characters in-universe have no concept called the Sentinel Feat.
Rather, they experience its application in the fiction. Most of the rules and features in the game do not correlate to tangible things in the fiction of the universe. Things like spells do, somewhat, but the Sentinel Feat translates to “this person is really good at pinning you down in a combat” – and this is something that is learned through being pinned down in melee combat.
Now, if a character with the Sentinel Feat was widely known for being good at pinning people down in melee combat, enemies who had heard of this character’s skills might be able to strategize against this tactic.
In response to this question: Is there a way to ask in game (i.e. in a non-meta way) what a character’s class is?
, T.J.L. nicely lays out the distinction between game mechanics and the fiction of the world:
class is a metagame construct: from the characters’ perspective, it doesn’t exist. There is no good way to determine “class” as a hard fact for the character, because a particular set of abilities does not cleanly map to the character’s identity and societal position in-world.
To put it a different way (using D&D 5E terminology)…
You have a two characters who wear heavy armor without discomfort, swing a greatsword with skill, and call on the powers of a deity to enhance their abilities and destroy their enemies. Are you dealing with a War Cleric or a Paladin?
You have two characters who wear medium armor, wield a longsword, and cast arcane spells. Are you dealing with an Eldritch Knight Fighter, or a multi-classed Fighter/Wizard?
To emphasize the difference even more… all four of these characters, if asked in game (without metagaming), may call themselves a knight.