If the mesh origin is sphere center.
For a perfect sphere with its origin (0, 0, 0) in its geometric center then the coordinate of any point is in the same direction as the normal.
As an example consider the cube with solidify modifier applied, its original faces are the outer skin, the new the interior, one method to test would be via a dot product being ...
Changes made in edit mode are not reflected in mesh data until changing back to object mode . How can I check face selection in edit mode If you do not wish to toggle the mode, consider using an edit mode bmesh, especially if the areas are going to be used to update the mesh.
As an example thought I'd add a numpy version, as well as ...
If it's round enough, you could add a sphere like so:
Merge the meshes, then box select all around it.
Deselect the new mesh with Shift-L with your mouse over it.
Shift-H to isolate selected. Now you can face select everything:
Maybe note-worthy, I had some limited success with Select Similar -> Coplanar.
Blender has tools to do it without decimating as in case of more complex models this changes geometry and should be used only if it is intended. Alternative way is to zoom in with e.g. the mouse wheel prior to selection. Or, in Edit mode, press A to select all.
Normally, in edit-mode you can select connected faces via Ctrl+Num +. This is called growing selection and works for vertices and edges as well. With Ctrl+Num - you can shrink your selection accordingly.
Since Ctrl+L does not work, growing selection will not work either, since your faces clearly are not connected. Maybe you ripped them accidentally. Try ...