As @Jaime Valencia mentions, you can use the Call Detail Record Analysis and Reporting Tool (CAR Tool) to view Call Detail Records to see if there has been any recent activity. (Google "Cisco CAR Tool" and read up a bit. Feel free to ask questions here about what you read.
If you find activity in the CDRs, that will show MACs and/or Device Names.
In CUCM navigate to Device > Phone and do a search for "Last Active" to see what devices might still be using your CUCM (registered).