PM me for a complete description of the Airfoil Performance Monitor (APM), which is the device you're talking about.
It works by using high-speed sampling of the airflow and then looking for the ratio between steady-state airflow and turbulence. At the stall on a clean wing, the ratio of the two will be clearly defined, and the wing will stall at nearly that same ratio whether clean or contaminated.
Works on the takeoff roll above about 50-60 knots.
Tested on lots of different aircraft and wind tunnels and does exactly what we claim.