As part of an effort to systematically improve our junior-level E&M I course, we have developed a tool to assess student conceptual learning of electrostatics at the upper division. Together with a group of physics faculty, we established a list of learning goals for the course that, with results from student observations and interviews, served as a guide in creating the Colorado Upper-Division Electrostatics (CUE) assessment. The result is a 17-question open-ended post-test diagnostic (with an optional 7-question pretest) and an accompanying grading rubric. We present measures of the validation and reliability of the instrument and grading rubric, plus results from 535 students in both standard and interactive-engagement courses across seven institutions as a baseline for the instrument. Overall, we find that the CUE is a valid and reliable measure, and the data herein are intended to be of use to researchers and faculty interested in using the CUE to measure student learning.