This condition is called a stall . On the slide shown above, the flow conditions for two airfoils are shown on the left. The shape of the two foils is the same. The lower foil is inclined at ten degrees to the incoming flow, while the upper foil is inclined at twenty degrees. On the upper foil, the boundary layer has separated and the wing is stalled.
Engineers usually rely on wind tunnel tests to determine the stall point.
But this must be done very carefully, matching all the important physical parameters. The plot at the right of the figure shows how the lift varies with angle of attack for a typical airfoil. At low angles, the lift is nearly linear. Notice on this plot that at zero angle a small amount of lift is generated because of the airfoil shape. If the airfoil had been symmetric, the lift would be zero at zero angle of attack.