The effects of a boundary layer turned turbulent are an increase in drag due to skin friction.
As speed increases, the upper surface transition point tends to move forward.
As the angle of attack increases, the upper surface transition point also tends to move forward.
The exact position of the transition point is hard to determine due to it being dependent on a large amount of factors.
Most of these methods revolve around analysing the stability of the (laminar) boundary layer using stability theory: a laminar boundary layer may become unstable due to small disturbances, turning it turbulent.
The eN method works by superimposing small disturbances on the flow, considering it to be laminar.
The assumption is made that both the original and the newly disturbed flow satisfy the Navier-Stokes equations.
This method assumes a flow parallel to the boundary layer with a constant shape, which will not always be the case in analysis.
Here the circular frequency ω is taken to be the real in the disturbance stream, and the wave number α complex.
Hence, in the case of an instability, the complex part of the wave number needs to be positive for there to be a growing disturbance.
For clean wind tunnels and for atmospheric turbulence, the critical amplification factors equals 12 and 4 in that order.
Experiments have shown that the largest factors affecting the position where this happens are the shape of the velocity profile over the lift-generating surface, the Reynolds number, and the frequency or wavelength of the disturbances itself.
[1] Behind the transition point in a boundary layer the mean speed and friction drag increases.