How to evaluate optical circle which are filled?

I am working on OMR(optical mark recognition) evaluation system where i am getting subjective area(ROI) after scanned image processing. As bubble matrix is 10*10, length of the roll number is some fix constant, how can i make logic to evaluate filled circle? How can i differentiate circle and other object inside image?

Answer 1

[Image Analyst]

See my attached demo. Adapt as needed, like first cropping off handwriting, if necessary.