Useful Tips for Viewing Clipping in Raw

Clipping occurs when the colour values of a pixel are higher than the highest value or lower than the lowest value that can be represented in the image. Overbright values are clipped to output white, and overdark values are clipped to output black. The result is a loss of image detail.

• To see which pixels are being clipped with the rest of the preview image, select Shadows or Highlights options at the top of the histogram. Or press U to see shadow clipping, O to see highlight clipping.
• To see only the pixels that are being clipped, press Alt (Windows) or Option (Mac OS) while dragging the Exposure, Recovery, or Blacks sliders.

For the Exposure and Recovery sliders, the image turns black, and clipped areas appear white. For the Blacks slider, the image turns white and clipped areas appear black. Coloured areas indicate clipping in one color channel (red, green, blue) or two color channels (cyan, magenta, yellow).

http://help.adobe.com/en_US/Photoshop/11.0/WSF1B29B95-2419-4b50-A058-A5FE2D7E5E6F.html