I put together a bash script to check my license usage. I'm going to run it daily in a cron job to send me e-mail alerts when I go over my license. You could modify it to do whatever alerting you want.
The script can be found here http://pastebin.com/QA2rzAXV
What about the eval version - some of my alerts are working - some aren't!
Disabled in Free License.
You could however wire up your own custom alerting system using the REST API.
It is a pitty, I'm used to test my scripts on a Test Machine with free Licence.
(The Prod Server is by our provider)
Alerting and real-time searching isn't available in the free version
For additional information, also see http://docs.splunk.com/Documentation/Splunk/latest/Admin/MoreaboutSplunkFree.