Displays current volume shadow copy backups and all installed shadow copy writers and providers. The vssadmin tool is a command-line utility to interact with the VSS.
vssadmin list writers
Lists all writers in the system
vssadmin list providers
Lists all currently installed shadow copy providers
more info on MS Technet website: http://technet.microsoft.com/en-us/library/cc754968%28v=ws.10%29.aspx