We advocate the use of property-based testing in the area of sensor networks and present a framework to apply this testing methodology. Our framework provides an expressive high-level language to specify a wide range of properties, starting from properties of individual functions to network-global properties, and infrastructure to automatically test these properties in Cooja, the network simulator of the Contiki operating system. We demonstrate the ease of use and effectiveness of our framework by two case studies. In the first, we test whether the energy consumption of the radio duty-cycle protocol X-MAC is within some specific bound. Property-based testing finds minimal network configurations where a small number of nodes violate the property. Property-based testing also reveals that the same property is not violated when ContikiMAC is used instead, but finds cases where ContikiMAC has higher energy consumption than X-MAC. In the second case study, we test the C API of CONTIKI's TCP socket library and find bugs in its event system that would be very hard to detect with other methods.