I think you are missing the point. To be of any use, the device must warn of an impending stall. Once the stall has occurred, the plane is falling and there is no mistaking that the stall has occurred. When operating near, but not yet at the stall angle of attack, there is still lift (i.e. a normal pressure differential between upper and lower surfaces.)

As far as a vane to sense stagnation point, on a wing sized for a .40-. 60, airplane the movement would likely be somewhere around 1/16 inch. Though such a small and precision device might be made, it’s not likely to be rugged enough to withstand the rigors of model airplane operation.

Another possibility might be a vane mounted on a boom near the wing tip that would align itself with the relative wind. Such a vane is frequently used during flight test of full-scale aircraft and would be a little simpler to construct and calibrate than a stagnation point sensor. It would still require precision (sort of watchmaker) skill in fabrication and would be a challenge to be made rugged enough for routine flight. Sometimes a ball is used on the end of the boom rather than a vane. The ball has two holes about 90 degrees apart connected to a differential pressure sensor. The sensor can be correlated with angle of attack. The instrumentation normally used costs many hundreds of dollars but perhaps with some ingenuity something could be adapted to work.

From your signature line you might be talking about model turbines. If so, the boom could be mounted directly in front of the nose, and the larger size would permit a little more room for mechanism.

Although as an academic exercise the subject is interesting, It seems as though the craft you are describing might be more akin to a military RPV rather than an around the patch fun machine. All of these problems have been addressed and solved in the commercial arena, and you may find all you need to know in unclassified, published sources.