I don't think there's really a way to assess a student's understanding of a subject. There's a difference between memorizing facts and actual understanding of information. For example, I received an A in my AP Bio class but don't remember anything about Biology. I think the best way to assess understanding is having the student use his/ her knowledge in real-life situations. I saw small attempts at doing this at my high school. They started to implement "common core" which tries to engage students in critical thinking in all subjects.