Quote: "Ctrl-n doesn't always work for me, that's why I do it manually."
it normally works so long as there are no doubles or internal faces. pressing ctrl+shift+n will recalculate the normals inside. holes are easy to see in uv mode, with display set to textured. assuming the mesh is not already textured, do a simple project from view unwrap, add->image->uv test grid. set the size to about 1024. any incorrect normals will show as a hole in the mesh. anouther way is to add a sub-serf modifier, incorrect normals will make an ugly black band.