Does the defensive duelist feat allow PC to know whether using AC boost would 100% avoid hit?

@EddieLaRusic Q: does the defensive duelist feat allow PC to know whether using AC boost would 100% avoid hit? “potentially” throwing me off yes – the player hears the attack result, then decides to use the feat

— Mike Mearls (@mikemearls) April 29, 2016

@EddieLaRusicthanks! The word “potentially” in the feats description makes it more confusing than it needs to be! yeah, the wording is kind of weird

— Mike Mearls (@mikemearls) April 29, 2016