Autonomous collaborative networks of devices are emerging in numerous domains, such as self-driving cars, smart factories and critical infrastructure, generally referred to as IoT. Their autonomy and self-organization makes them especially vulnerable to attacks. Thus, such networks need a dependable mechanism to detect and identify attackers and enable appropriate reactions. However, current mechanisms to identify adversaries either require a trusted central entity or scale poorly.In this paper, we present SADAN, the first scheme to efficiently identify malicious devices within large networks of collaborating entities. SADAN is designed to function in truly autonomous environments, i.e., without a central trusted entity. Our scheme combines random elections with strong but potentially expensive integrity validation schemes providing a highly scalable solution supporting very large networks with tens of thousands of devices. SADAN is designed as a flexible scheme with interchangeable components, making it adaptable to a wide range of scenarios and use cases. We implemented an instance of SADAN for an automotive use case and simulated it on large-scale networks. Our results show that SADAN scales very efficiently for large networks, and thus enables novel use cases in such environments. Further, we provide an extensive evaluation of key parameters allowing to adapt SADAN to many scenarios.