These are a great couple of materials you can use for checking the following:

* Check Normal Orientation renders Polygons with inverted normals in pink. Helpful for models with huge polygon count
* Check Overlap helps finding holes, hidden faces, and duplicate faces
* Check Reflection Angles maps Fresnel angles to colors. Stops guesswork for Fresnel ranges
* Check Reflections displays a number of stepped gradients on the model. Great for checking surface continuity
* Check UV displays the UV mapping with my UC Check image
* Render Normal Map renders out the surface normal for the use as normal maps

For more information or to download check out: http://news.kilibee.com