Have you check out the DeploymentMonitor app. It ships with Splunk, but is not enabled by default.
To enable it, go to Manager -> Apps and click 'enable' for the SplunkDeploymentMonitor
Then you can choose DeploymentMonitor in the 'Apps' menu (it will not have much data to show right away, since it works with scheduled searches). You can set up alerts for 'quiet' or 'missing' forwarders.
Hope this helps,
Note that as of version 5, DeploymentMonitor has to be downloaded separately. This can be done through the Manager. /k
Check out this previous Splunkbase answer. You can create an alert based on that search. You'll just need to adjust the time to be 'age > 1800' for 30 minutes.
| metadata index=main type=hosts | eval age = now()-lastTime | where age > (2*86400) | sort age d | convert ctime(lastTime) | fields age,host,lastTime