In the Image editor, If you Right click on the image, you can sample the pixels of the image. A small window appears at the bottom of the screen where you can read the pixel information.
(for previous versions of blender: left click on the UV/Image editor window)
From Left to right you see:
$X$ (horizontal) and $Y$ (vertical) pixel position ($0$,$0$ is at the bottom left of the screen)
Z depth, in blender units.
Red, Green, Blue, Alpha information before the color management transforms (Scene Referred, Linear values).
Red, Green, Blue after the Color Management transforms (Display Referred, values)