Try this:
For each object find center point “P0” (centroid, with “Volume” component).
For each “P0”, find closest point to surface, “P1”.
Evaluate surface from there and find local normal vector, “N”.
Create vector from P0 to P1 , “V”.
Calculate angle from N to V, if angle is greater than pi/2 object is on back side of surface, otherwise in front side.