Passive testing is a technique that aims at testing a running system by only observing its behavior without introducing any test input. The non-intrusive nature of passive testing makes it an appropriate technique for interoperability testing, which is an important activity to ensure the correct collaboration of different network components in operational environment. In this paper we propose a passive interoperability testing approach, especially for request-response protocols in the context of client-server communications. According to the interaction pattern of request-response protocols, the observed interactions (trace) between the network components under test can be considered as a set of conversations between client and server. Then, a procedure to map each test case into these conversations is carried out, which intends to verify the occurrence of the generated test cases as well as to determine whether interoperability is achieved. The trace verification procedure has been automated in a passive testing tool, which analyzes the collected traces and deduces appropriate verdicts. The proposed method and the testing tool were put into operation in the first interoperability testing event of Constrained Application Protocol (CoAP) held in Paris, March 2012 in the scope of the Internet of Things. By using this approach, an amount of CoAP applications from different vendors were successfully and efficiently tested, revealing their interoperability degree.