People learn whenever and wherever possible, and whatever they like or encounter--Mathematics, Drama, Art, Languages, Physics, Philosophy, and so on. With the bursting of knowledge, evaluation of one's understanding of conceptual knowledge becomes increasingly difficult. There are a lot of demands for evaluating one's understanding of a piece of knowledge, e.g., facilitating personalized recommendations; discovering one's expertises and deficiencies in a field; recommending topics for a conversation between people with different educational or cultural backgrounds in their first encounter; recommending a learning material to practice a meaningful learning etc. Assessment of understanding of knowledge is conventionally practiced through tests or interviews, but they have some limitations such as low-efficiency and not-comprehensive. We propose a method to estimate one's understanding of conceptual knowledge, by keeping track of his/her learning activities. It overcomes some limitations of traditional methods, hence complements traditional methods.