Detecting Denial of Service (DoS) attacks and reducing the energy consumption are two important and frequent requirements in Wireless Sensor Networks (WSNs). In this paper, we propose an energy-preserving solution to detect compromised nodes in hierarchically clustered WSNs. DoS detection is based on using dedicated inspector nodes (cNodes) whose role is to analyze the traffic inside a cluster and to send warnings to the cluster-head whenever an abnormal behavior (i.e. high packets throughput) is detected. With previously introduced DoS detection schema cNodes are statically displaced in strategic positions within the network topology. This guarantees a good detection coverage but leads to quickly draining cNodes battery. In this paper we propose a dynamic cNodes displacement schema according to which cNodes are periodically elected among ordinary nodes of each atomic cluster. Such a solution results in a better energy balance while maintaining good detection coverage. We analyze the tradeoffs between static and dynamic solutions by means of two complementary approaches: through simulation with the NS2 simulation platform and by means of statistical model checking with the Hybrid Automata Stochastic Logic.