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Another method for teasing out the sky conditions from weather file data is to look at the ratio of direct normal to diffuse horizontal flux (can use radiance, irradiance, or illuminance). If you have a really high direct normal and low diffuse horizontal, you have a clear sky, and vice versa -- which follows, since a low direct normal means there are clouds between the sun and the detector, and all that flux is getting scattered by the clouds. The following images show these distributions, using Seattle, WA and Boulder, CO as exemplars for cloudy and clear:

These are falsecolor visualizations of June 21st at noon, using the Perez sky model; gendaylit was used to make the skies. The commands to create the two skies are underneath the images, and the values after the "-L" are the direct normal and diffuse horizontal illuminance (lux), respectively. This is admittedly a somewhat indirect means to get what you're looking for, but generally speaking, weather files have those two pieces of info in them...