The only way really to tell is by the logs in VC (assuming it wasn't removed and re-added). So you can click on a VM, look at the tasks/events for that VM, and see when it was "powered on" last.
Then you can start the VM (if this is Windows) look for last updated folders, usually under documents and settings and see if you can tell if files were last modified..
Then you can look in the event viewer for that machine, and see if you can look at either audit or security, and that should give you a list. Obviously if you login you have to go back a few entries to see when someone last accessed it, and that may give a clue.
This is a tough thing to do, and I have been exploring better ways to find out this info myself... but alas, there doesn't seem to be an easy way to tell when a machine was ultimately used or last powered on realistically.. it's a manual process.
