Wireless networking relies on a fundamental building block, neighbor discovery (ND). The nature of wireless communications, however, makes attacks against ND easy: An adversary can simply replay or relay (wormhole) packets across the network and mislead disconnected nodes into believing that they communicate directly. Such attacks can compromise the overlying protocols and applications. Proposed methods in the literature seek to secure ND, allowing nodes to verify they are neighbors. However, they either rely on specialized hardware or infrastructure, or offer limited security. In this paper, we address these problems, designing a practical and secure neighbor verification protocol for constrained Wireless Sensor networks (WSNs). Our scheme relies on estimated distance between nodes and simple geometric tests, and it is fully distributed. We prove our protocol is secure against the classic 2-end wormhole attack. Moreover, we provide a proof-of-concept implementation with off-the-shelf WSN equipment: Cricket motes.