What you know as a player and what your Character knows are often different things in Fate. If the NPC is using his "Enraged and Insane" aspect and is acting all crazy, you could probably argue that your Character realizes he's lost his cool. If he's Enraged and Insane, but keeping outward appearances, you'd need an Assess in my book.
But this is very situational. It is not generally true that the PC knows what the Player knows. You helped create the world after all.