You can check whether your hg-agent is reporting metadata to Hosted Graphite via the Agent list, and from there you can use the Base Metrics dashboard to see if metric data is flowing in. If not, you may need to dig a little deeper.
Logs are in /var/log/hg-agent: you can see the metrics generated by diamond in archive.log, as well as the various daemon logs supervisord.log, diamond.log, periodic.log, and forwarder.log e.g.
$ tail -f /var/log/hg-agent/forwarder.log
Process structure and other details are described in System Layout.
You can inspect the agent as a service like any other, e.g. on Ubuntu trusty:
$ sudo service hg-agent status
If you’d like to look at things from the agent’s supervisor point of view:
$ sudo /opt/hg-agent/bin/supervisorctl --config=/etc/opt/hg-agent/supervisor.conf status