You can use the CommandDispatcher to invoke a Command on the cluster that returns whether a given node is the active provider of some singleton service.  e.g.

public class IsMasterCommand implements Command<Boolean, ServiceRegistry> {
   private final ServiceName name;

    public IsMasterCommand(ServiceName name) {
       this.name = name;
   }

    @Override
   public Boolean execute(ServiceRegistry registry) {
       ServiceController<Singleton> controller = registry.getService(this.name.append("singleton");
       return (controller != null) ? controller.getValue().isMaster() : false;
   }
}