Wireless sensor networks cannot be secured against internal attacker with only cryptographic techniques because their nodes are not physically protected and can be easily captured by the attacker. Therefore, in this work we consider intrusion detection systems that monitor behavior of sensor nodes and detect the malicious ones among them. Our work is exploratory in that we propose metrics for evaluation of these systems. There are evaluation metrics for intrusion detection systems for conventional wired and wireless networks. However, to our best knowledge there is no work that proposes evaluation metrics for intrusion detection systems for wireless sensor networks. We divide the proposed metrics into two groups. In the first group there are metrics for detection techniques without a response mechanism. In the second group there are metrics for detection techniques together with a response mechanism.