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)
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)