There are 3 rules which help to identify the status of an agent:
Rule: Agent Health State
This rule checks that all agents are in a good health state.
Rule: Idle Agents
This rule checks periodically whether there are idle agents. An agent is considered idle if it's running but there's no associated data being seen by the server for IdleAgent.
Note: This rule is disabled by default and needs to be enabled. The rule is time driven and runs every 5 minutes.
Rule: Disconnected Agent Manager Clients
This rule checks whether all Agent Manager Clients (e.g. FglAM) on a particular host have disconnected from FMS.
Note: This is not for agents itself, it is for Agent Managers and checks if those are disconnected. If an Agent Manager is disconnected, the agents will not work either, the 2 rules above do not trigger if the Agent Manager is gone.