Constrained Application Protocol (CoAP) is an application protocol designed for the Internet of Things, where smart devices cooperate to provide machine-to-machine Web services. In this context, a high level of interoperability is crucial. This paper addresses the interoperability testing of CoAP applications. It proposes a methodology based on passive testing, which is a technique to test a running system by only observing its behavior without introducing any test input. The methodology (the proposed method and a corresponding testing tool) was put into operation during the CoAP interoperability testing event (Plugtest) organized by ETSI in Paris in March 2012, where a number of CoAP applications were successfully tested, showing the validity and efficiency of this approach.