How can I tell from the location of flow separation if an airfoil is stalled? I am in the process of completing a carbon airfoil for my track car (sedan) and I am looking to determine stall angle visually.  AKA, wool tufts and highway runs with my GoPro pointed upward from the trunklid.

Since I don't really know the angle of incidence at which the air coming off the rear window will hit the wing, visually looking for flow separation is really the only way for me to determine my max angle and work down from there based on the car's balance at the track.

Is there a paper (or common knowledge) regarding over what % of chord length flow separation must occur for the wing to be considered stalled?  Highly cambered airfoil, Selig S1223, 11.5" chord, 66" span.  I have searched for papers on the topic but cannot find anything.

Thanks!
The boundary layer separates usually at somewhere around 10-15% of chord near stall AOA (which is where you place vortex generators when using them to delay stall AOA).  Separation will start at the trailing edge and move forward.  If it was me I'd tuft the entire lower surface from just behind the LE, maybe 10% so you can really see everything that's going on.  Doesn't hurt to put too many on.