Run this command to see who has something checked out in TFS: 

“C:Program FilesMicrosoft Visual Studio 9.0Common7IDEtf.exe” status /server:SERVERNAME /user:*  $/TFSPROJECTBRANCH /recursive /login:DOMAINYOURUSERNAME,YOURPASSWORD

Change the parent branch by replacing the TFSPROJECTBRANCH with the name of an alternative branch.

For VS2005 only machines change Visual Studio 9.0 to Visual Studio 8.

For best results pipe the results out to a file as the results are wide and are hard to read in command window.