Here is a $C_L$ / $AoA$ curve that I took from Wikipedia.
The better textbooks say that a stall is that condition in which a further increase in angle of attack will result in a reduction of lift. The point at which that transition happens is known as the critical angle of attack. Theoretically, sustained flight is possible at angles beyond the critical angle of attack - take a look at the chart. If the airplane can sustain level flight at point $A$, it can sustain level flight at point $B$.
Is there a practical way that I can demonstrate sustained flight on the backside of the lift curve without an angle of attack indicator?
Or to ask the question another way,
Is there a practical way to tell when an airplane has exceeded the critical angle of attack without an AoA meter?