Applications over wireless sensor networks are growing quickly. Traditional software development tools are not well adapted to this technology. In particular, adequate testing methodologies are required. Many issues have not been formally addressed as energy-conservation, congestion control, reliability data dissemination, security. In this paper, we present a pragmatic approach to detect some kind of anomalies based on defined scenarios on wireless sensor networks. A scenario is considered as a set of events which should happen on the network in an ordered way. We discuss a formal architecture able to perform these scenarios over the wireless sensor networks and to raise alarms if necessary. This methodology has been implemented on a prototype and has been experimented with various examples. This contribution is a first attempt for a formal testing methodology for wireless sensor networks.