Governance copes with the heterogeneity, autonomy and diversity of interests among different agents in a multi-agent system (MAS) by establishing a set of norms. Most of the governance enforcement mechanisms usually check norm violations from the point of view of interaction protocols. However, in MAS, with the presence of heterogeneous and independently designed agents, there will be private messages, that will only be perceived by their senders and receivers, and execution of actions that will only be observed by the agents that execute them or by the agents that suffers from their consequences. This work presents a governance mechanism for MAS based on testimonies. Agents can witness facts that are related to norm violations witch they are aware of. The proposed mechanism is composed by three sub-systems: Reputation, judgment and sanction. This work focuses only in the judgment sub- system, witch is responsible for receiving testimonies and providing a decision, pointing out if an agent has really violated a norm. The judgment sub-system architecture and a generic judgment process will be presented. Finally, the use of this mechanism will be exemplified by a case study.