In the Unity ARKit plugin, you can get data about tracking points and surfaces. This allows you to create debug visualizations or even show the player where surfaces are detected and how big of an area was detected.
Is it possible to visualize a ground plane area with Vuforia?
With a PlaneFinderBehaviour set to AUTOMATIC, I guess you could draw a point at every HitTestResult for visualizing tracking points? But that would only be based on a single screen point at a time.