Web APIs following the REST architectural style (so-called RESTful Web APIs) have become the de-facto standard for software integration. As RESTful APIs gain momentum, so does the testing of them. However, there is a lack of mechanisms to assess the adequacy of testing approaches in this context, which makes it difficult to measure and compare the effectiveness of different testing techniques. In this work-in-progress paper, we take a step forward towards a framework for the assessment and comparison of testing approaches for RESTful Web APIs. To that end, we propose a preliminary catalogue of test coverage criteria. These criteria measure the adequacy of test suites based on the degree to which they exercise the different input and output elements of RESTful Web services. To the best of our knowledge, this is the first attempt to measure the adequacy of testing approaches for RESTful Web APIs.