Select a surface of brep

Dear all,

I am working on a uni assignment using the voronoi 3d  to make an arc of voronoi cells. I would like to create opening (holes) on  my surface modulated by attractor curve or points.

My question is how do I select surfaces of the interior or exterior. My plan is the exterior surfaces are mostly opaque while the interior have holes. I was trying to see if I can find a way to get the surface normal?? to point towards a particular point attractor or curve attractor (in red) and the exterior surfaces directions (in blue arrows) are just radiating out. 

Any suggestion will be greatly appreciated. 

Thank you in advance.