Previous research on the security of network coding focuses on the detection of pollution attacks. The capabilities of network coding to detect malicious attacks have not been fully explored. We propose a new mechanism based on physical layer network coding to detect the Sybil nodes. When two signal sequences collide at the receiver, the starting point of the collision is determined by the distances between the receiver and the senders. When the distance between two receivers is large enough, they can combine their interference sequences to recover the original data packets. On the contrary, the Sybil nodes attached to the same physical device cannot accomplish the data recovery procedure. We have proposed several schemes at both physical and network layers to transform the idea into a practical approach. The investigation shows that the wireless nodes can effectively detect Sybil nodes without the adoption of special hardware or time synchronization.