I consider myself to be a "reflective teacher". I look at the output of my students' work to determine whether or not I have conveyed the lesson to their understanding (within reason, of course).
As much of a cliche the word "data" has become in education, I truly have an appreciation for the data gathered from different testing (i.e. end-of-the-unit tests and yes, even the dreaded standardized testing). I do believe studying the data helps me to narrow down the skills my students are missing, and determine where I need to re-teach or present information in another manner.