Differentiation and Assessment

The readings for this week have me thinking about how I can better differentiate my assessments to meet the needs of all my students and to better assess them. I have one student, "M.S.", who seems flighty. She is forgetful, when she is called on she mostly gives a blank stare, and on the surface she seems to not get it. But there are times when she will give in depth answers to questions that leave me wondering where they came from. How can I better assess her and is there a way I can pull out more of those moments of brilliance?