In a wireless sensor network (WSN), the sensor nodes (SNs) generally localize themselves with the help of anchors that are pre-deployed in the network. Time of Arrival (ToA) is a commonly used mechanism for SNs localization in WSNs. In ToA, the SNs localize themselves using the positions of the anchors and the time difference between the receipt of a radio and ultrasound signal transmitted by each anchor. In this setting, the localization process has a high risk of being subverted by malicious anchors that lie about their position and/or distance from the SNs. In this paper, we propose an efficient scheme that helps identify and revoke these malicious anchors. We use a mobile verifier (MV) that moves throughout the network, in some pre-determined manner, and obtains multiple location references from each anchor. For each anchor, the MV tests the mean and the variance of the collected sample to identify if the anchor is malicious. We show through simulations that our scheme successfully identifies more than 80% of malicious anchors with less than 60 references from each. Also, the percentage of false positives is close to 0.