Interoperability describes the ability of engineering tools to collaborate across tool borders, organizational borders, and workflow phases. Therefore, it is considered as an important indicator for engineering efficiency, but is supported by today's industrial software only to some extent. The authors propose a novel method for the assessment of automation engineering tools, regarding their possibility to provide data for other software tools and to make use of the data provided by other tools. The authors have developed a metric to systematically assess and compare the openness of engineering tools. Thus, users can estimate better and for the first time based on objective criteria whether an engineering tool is suitable for a particular need within an engineering use case, and suppliers might find interesting hints towards possible improvements.