Currently the only way to find out which agents are running builds is to query the running builds, then loop through those build ids to find out which agent is running that build. There's no way to just ask for a list of running agents. Even being able to get this info off querying the agents directly would be helpful, instead all you get back from an agent query is the agent id, name, and url. This is a big flaw in the REST API for me as my use case is automatic adding/removing of build hosts.
I would love it if something like "http://teamcity:8111/httpAuth/app/rest/agents?locator=running:true" could be implemented.