If you have applied materials to the backs of faces models the wrong way round there are also tools like 'FixReversedFaceMaterials' [do a search] - that tool finds all faces in a selection that have a material on the back but not the front, it then reverses them and swaps the material from back to from keeping the UV mapping so any textures appear exactly the same as they were before, but now all faces are hopefully correctly oriented and 'materialed'... Sometimes simply reversing faces to be consistently oriented won't work without lots of subsequent reapplication of textured materials and UV mapping adjustments - this does it in one step...