The latest trend across different industries is to move towards (open) web APIs. Creating a successful API, however, is not easy. A lot depends on consumers and their interest and willingness to work with the exposed interface. Structural quality, learning difficulty, design consistency, and backwards compatibility are some of the important factors in this process. The question, however, is how one can measure and track such attributes. This paper presents the beginnings of a measurement framework for web APIs that is based on the information readily available both to API providers and API consumers - API call responses. In particular, we analyze the tree-based hierarchical structure of JSON and XML data returned from API calls. We propose a set of easy-to-compute metrics as a starting point and describe sample usage scenarios. These metrics are illustrated by examples from some of the popular open web APIs.