How to access shadow map depth values?

I would like to use the depth values from a shadow map to create a blanket mesh over the scene for various effects. I've found that I can access the attenuation (0 or 1) using LIGHT_ATTENUATION, but I need the actual depth stored in the texture. How do I access that?

Does the method of access differ for different types of lights? If so, I'm primarily interested in directional lights.