Beyond the point, made by others, that a spirit level will not give you the airplane's attitude (on account of acceleration), it is also the case that the airpl

Beyond the point, made by others, that a spirit level will not give you the airplane's attitude (on account of acceleration), it is also the case that the airplane's attitude does not tell you the angle of attack, which also depends on the airplane's vertical speed with respect to the airmass. Pilots-in-training are taught not to rely on the airplane's attitude (normally judged, in visual flight, by the position of the nose or top of the instrument panel relative to the horizon) as an indicator of whether a stall is imminent. In a steady state, and for a given wing configuration (flaps/slats/spoiler positions), there's a causal relationship between angle of attack and speed (which also depends on air density, aircraft weight and load factor (how many gs you are pulling)), such that the indicated airspeed (which measures ram air pressure in a pitot tube, and fortuitously cancels out the density dependence) has historically been the proxy for angle of attack - and, at least in small airplanes, it still is. Even small airplanes often have a separate stall warning device, which is triggered by the movement of the leading-edge stagnation point (where the flow divides between going above and below the wing) as the angle of attack changes. In its simplest form, it is operated by a small tab which is flipped up when the stagnation point moves below it.