The Normals command draws 3D Lines with normal (perpendicular) vectors of selected 3D Faces. You can choose to either draw unit normals (from the 3DFACE center outwards) or recolor the selected faces to blue or red.

The FlipFace command flips selected 3DFaces changing their normal. You can easily change backfaces to normal faces without changing their geometry (3D position, size, vertices).