To know whether a student writes with good grammar and spelling, sample the student's writing. To check arithmetic skills, give a quiz or assign a problem set. But understanding proves more subtle. Certainly it does not reduce to knowledge. Understanding what Magellan did or what Newton's first law means calls for more than just reproducing information.
Understanding also is more than a routine well-automatized skill.
The student who deftly solves physics problems or writes paragraphs with topic sentences may not understand much about physics, writing, or what's being written about at all. While knowledge and skill can be translated as information and routine performance on tap, understanding slips by these simple standards. In a phrase, understanding is the ability to think and act flexibly with what one knows. Learning facts can be a crucial backdrop to learning for understanding, but learning facts is not learning for understanding. Please provide us with feedback on this site.