Detecting blank image files

Use the identify feature of ImageMagik CLI as given here:

http://www.imagemagick.org/script/identify.php

With command:

$ identify -format "%#" source.png

If the number of colors is 1, you have a blank page.

You can also use the command:

identify -verbose source.png

The standard deviation, skew and kurtosis will be 0 for a blank image.