I am confidant that no friends of mine would ever use levels to adjust contrast or color, but seeing how this blog is spreading like wildfire (1 e-mail so far), someone out there just might. This is just a very small tip for using levels to let you see when clipping (blown out highlights, or blocked up shadows) starts to occur.
While moving either the shadow or highlight sliders, hold down on the alt key (win) or the option key (mac) and you will be able to preview the clipping. If you see red in areas that means that the red channel will have no information there. If you see white or black then all three channels (red, green, and blue) are blocked up.
I use this preview approach in the camera raw dialog box but did not know that it also worked on levels.
I tried it on curves as well but no luck.