There is no Inbuilt way to do this. A z-aware polygon, is fundamentally not 3 D in itself. What is Z Aware are the vertices. So for a point inside the polygon (i.e. inside in the sense of X-Y domain) no Z value is defined.
(If you having problems, visualizing this, imagine a z aware polygon that does not lie in one plane. Some points are on that plane, some above and some below. For this polygon, only the vertices have a defined z value, and not any points in the interior.)
So you can proceed in several ways:
- If you are assuming that the vertices are sort of spot heights, then the inside of the polygon is the surface in between the points is the 3 dimensional surface which is defined by these spots heights.
- Another way would be to convert this geometry into a Multipatch, where the surfaces are defined as triangles. There you can find the z value at any point.