Communication neighborhood in wireless sensor networks changes often as links break or appear. Therefore, monitoring link quality and (logical) network topology is necessary. As node placement has a large influence on the radio neighborhood and its changes, different positions should be evaluated before starting the actual application. In this paper we introduce Sensorium, an active monitoring system that supplies the user with an insight into the neighborhood relations between nodes and their changes in time. It can be used before the actual deployment to evaluate different possibilities of node placement and choose the one that offers the best connectivity. Key words: wireless sensor networks, active monitoring, robustness, Sensorium