Good inhomogenity measures in images?
The variance is a good inhomogenity measure. 
For example, take 20*20 patches and calculate the empirical variance of the patch using these 400 samples.
For very simple purpose, this could work.

Edit : After viewing your example images, try to just play around with simple tools like manual thresolding and edge detection.

Edit 2: Added Example with different patch sizes (40, 10, 2)