Sensor networks have increasingly become the subject of intense scientific interest over the past few years. In this work we focus on detecting intrusions in sensor networks. The intrusion detection community has been focusing mainly on wired networks. But techniques geared towards wireline networks would not suffice for a sensor environment because of the constraints associated with such networks. In this paper we consider arbitrary sized sensor networks and propose algorithms to improve the detection rates by intelligently enabling the intrusion detection functionality on particular sensor nodes. The proposed algorithms, based on the concepts of minimum cut-set and minimum dominating set, allow for a distributed implementation. The performance of the algorithms in identifying intrusions using signature based detection techniques is studied via simulations.