With the free AutoLISP utility CADstudio Normals you can draw normals of 3D faces in your DWG drawing.
Download the Normals.vlx from www.cadstudio.cz/download, load it with APPLOAD and start the NORMALS command. Now select all 3DFaces to analyze (you may want to EXPLODE your 3D mesh object first, e.g. objects from 3DPLOT). You can choose to draw Normals or to recolor the individual Faces.
The Normals option draws vectors going from the face center "outwards", from its "front" side. Any normals going away from you indicate a backface (reversed). The normals are created in the layer "Normals", they have the same length (1/20 of the current viewport size) and they are either red or blue. Red normals go towards the current view direction (camera), the blue ones face away. Please note that the colors depend on the current view direction. Rerun the command if you change the view rotation.
The Faces option modifies your 3D model (faces) so that the faces facing away from the current camera will be blue ("dark side"), the other will be red (you should only see red faces in shaded view styles, blue ones are backfaces).
You can use this tool to analyze (and possibly flip) backfaces, not rendered with one-sided materials.
Missing "fur" (going inwards) and blue areas indicate a backface problem in this sphere model from 3DPLOT (both options of the NORMALS command were used):
You can easily flip face normals using the FlipFace utility - see the Tip 9063.