I also disagree that we should not "test" students in the traditional sense. To no do so separates us from the rest of the school, gives the impression our subject is not worth testing, and that we are somehow less important. We can have a separate discussion about testing and how valuable it is or is not--I'd prefer NEVER to have to test my students, BUT it is the reality within we live and work. (That may change but let's stick with reality)
...and what is wrong with testing vocabulary terms like in an English class? Shouldn't we test the historical knowledge we have passed on to our students like they do in history classes? If we use rulers, would it be unusual to do an assessment on measuring? What we do in art can and does support all other curriculum... if you allow it to.
I'd argue that art is THE most important core subject, helping all other subjects make more sense at a deeper experiential level. It's why art students score 100 points higher on things like the SAT.
I believe we should not test for "creativity" or "prettiness," but we should grade and assess vocabulary, elements, principles, and where appropriate, the core concepts we incorporate into our lessons.
When we grid, measure, and draw—we use geometry. When we make sculptures—we use engineering. When we mix colors—we reveal information about physics. When we create illustrations for stories—we learn about literature. When we review the styles of art from da Vinci to Bansky—we teach history. When we write about art—we strengthen these skills. When we create works of art, we solve complex visual problems in creative ways. Art is the meeting place of all subjects.
I won't bore anyone with more, but you can read a bit more HERE.
Looking for assessments? Free ones HERE, others are HERE.