Several models have been developed to measure creativity. The heuristic framework for creativity measurement is a typical example. You can also use those models to measure creativity in art. This link could be of help: http://www.senseandsensation.com/2012/03/assessing-creativity.html
The attached article provides insights and lessons learned from using measures of innovative activities for scholars who are interested in capturing creative activities. I can strongly recommend it as a source for all your efforts you might undertake in your own learning&teaching environment to measure creativity.
The Creative Arts tab above has similar questions and answers that can be helpful for you: https://www.researchgate.net/topic/Creative-Arts?ev=tp_pst_dtl_xkey
I like the link by Harry Barton Essel. Working with students with severe physical impairments, I look for ways to modify tasks to increase student engagement and participation. The link makes me want to investigate my own drawings in depth (then again, maybe not!).
I think that successful evaluation is directed to skill tests that depend on observation forms and depend on the time and mastery scales in performing the skill of art.