A couple of things spring to mind for such a task.
- Create a copy of the image and convert to grey-scale
- Run an edge-detect filter on it (this will highlight the differences in contrast at feature boundaries
- I vaguely recall reading of an adaption to the Hough transform that allows circles to be detected, rather than the most common straight-line
Yup, Circular Hough Transform appears to be the way to go. Plug that into google, make a pot of coffee and let the 'fun' and games begin!
Hough Transform Circles