Hi Osama, if they multi-agent system is composed of cognitive agents, such as BDI agents for example, you could follow the Prometheus Methodology which details a process for identifying agents. There are others such as Tropos or OMase and the paper attached which I've coauthored provides an overview of these three methodologies with a case study. Hope this helps
Article Using three AOSE toolkits to develop a sample design
Hi! I agree with John that different AOSE methodologies can guide you to define the necessary types of agents in a multiagent system. Most of the methodologies have shown that there is a conceptual level for analyzing agent-based systems that should describe the external view point of agents by the Role Models (the characteristics/task of each agent) and the Society Models (the relationships and interactions between agents).