In wireless sensor networks (WSNs) deployed in an outdoor environment, obstacles may occur as a result of a non-uniform distribution of the sensor nodes or the presence of barriers. These obstacles result in a degradation of the network performance, so obstacle identification is a major concern in most WSN applications. This paper develops a Decentralized Boundary Detection (DBD) algorithm for identifying sensor nodes near a hole or obstacle in the WSN. The algorithm does not require any knowledge of the node locations or distances between two nodes. The detection capability is provided even in networks where the sensor nodes have a non-unit disk communication range.