How we can say that flow at lower Reynolds number are also turbulent?
What are the certain parameters which we look for before saying that the flow is turbulent?
Is the flow over a post-stall airfoil is fully turbulent (while starting from its leading edge) at moderate Reynolds number if flow separation is starting just from the leading edge?
Turbulence is characterized by three-dimensionality, unsteady behaviour and appears characterized by a large amount of vortical structures of different sizes. The more the vortices are bigger the more are energetic. Energy is transferred in inertial way up to the small vortices where dissipation acts converting kinetic energy in thermal energy in an irreversible way.
This behaviour appears by means of an extended spectrum of energy
I would recommend the answer of Prof. Filippo Maria Denaro for turbulence.
In the second part: One may expect turbulent to laminar transition if airfoil is pitching. Appearance of LSB is another thing which you should observe at leading edge. But If your flow is fully turbulent after post-stall and you are neither changing operating parameters nor geometric; your flow will remain turbulent.
"What are the certain parameters which we look for before saying that the flow is turbulent"? -- It depends on how you are analyzing the flow i.e. By experimental (HWA, PIV) or by numerical modelling (If RANS then transition models). In both the cases transition is captured by fluctuating quantities like: Cp, CL, Cd.
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