Physics inquiry can be interpreted as the construction of a cogent argument in which students apply inquiry knowledge and knowledge of physics to the systematic collection of relevant, valid, and reliable data, creating optimal scientific support for a conclusion that answers the research question. In learning how to devise, conduct and evaluate a rigorous physics inquiry, students should learn to choose and apply suitable techniques and adhere to scientific conventions that guarantee the collection of such data. However, they also need to acquire and apply an understanding of how to justify their choices and present an optimally convincing argument in support of their conclusion. In this modified and augmented Delphi study we present a view of inquiry knowledge and a way to assess it that acknowledges both of these components. Using our own expertise with teaching physics inquiry and using curriculum documents on physics inquiry, "inquiry knowledge" is deconstructed as a set of "understandings of evidence" (UOE)-- insights and views that an experimental researcher relies on in constructing and evaluating scientific evidence. While insights cannot be observed directly, we argue that their presence can be inferred from a student's actions and decisions in inquiry, inferred with more definitude as a more explicit and adequate justification is provided. This set of UOE is presented and validated as an adequate, coherent, partially overlapping set of learning goals for introductory inquiry learning. We specify conceivable types of actions and decisions expected in inquiry as descriptors of five attainment levels, providing an approach to assessing the presence and application of inquiry knowledge. The resulting construct, the assessment rubric for physics inquiry, is validated in this study. It distinguishes nineteen UOE divided over six phases of inquiry. Preliminary results suggesting a high degree of ecological validity are presented and evaluated. Several directions for future research are proposed.