This question should not only guide our teaching, but also our testing. Assessing learning is a vital part of teaching and tests, properly constructed, are an aid to learning. Two kinds of assessment concern us.
Traditional testing is concerned with the students can recall from what was taught.
The CAT as this is called, seeks to focus the student on his or her own learning processes as well as to provide feedback to professors before conventional testing takes place. For instance, one popular CAT is to ask students at the end of a class to write a one minute paper telling what they thought the most important point of the day's class was. This tells the professor whether or not his or her presentation was properly understood or not. It also makes the student think actively about the content of the class. A variation on this is to ask students what was the muddiest point or what material needed further explication.