We introduce the study of non-manipulable measures of manipulative behavior in multi-agent systems. We do this through a case study of decentralized finance (DeFi) and blockchain systems, which are salient as real-world, rapidly emerging multi-agent systems with financial incentives for malicious behavior, for the participation in algorithmic and AI systems, and for the need for new methods with which to measure levels of manipulative behavior. We introduce a new surveillance metric for measuring malicious behavior and demonstrate its effectiveness in a natural experiment to the Uniswap DeFi ecosystem.