Take a look at the license_usage.log on your license master. From the docs http://docs.splunk.com/Documentation/Splunk/6.2.1/Troubleshooting/WhatSplunklogsaboutitself
license_usage.log
Indexed volume in bytes per pool, index, source, sourcetype, and host. Starting in 4.2, license_usage.log is available only on a Splunk license master.
... View more