When assessing student artwork I look for evidence that they learned the concepts I set out to teach within that lesson.  I consider before I teach a specific concept what the best way for me to measure what they learned will be for that concept.  I often use rubrics to layout what specific criteria I am looking for from the students.  Within the rubrics I most often look for students to have acquired a skill, knowledge of design elements or principles, the ability to recognize artist, define vocabulary, and recognize art movements.  At the end of the term I also look over the students work to assess how much progress the student has made throughout the term.