I had posted here suggesting you use PIX, but then saw the opengl calls and hung my head in shame.
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Posted 25 July 2013 - 05:16 AM
If you're running on anything below windows 8 you can also do a frame capture with PIX (an invaluable debugging tool for graphics).
Once you've found the draw call where you use your shadow map, you can have a look at the textures that get passed in to the shader. PIX has a very useful feature that lets you mess around with the brightness of an image. As you can see in the image below it makes it much easier to see exactly what's going on.