Other users checkouts are not visible in Source Control Explorer in Visual Studio 2012, TFS is express edition :

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TFS 2012 introduces a new concept called local workspaces, which differ subtly from the behavior of workspaces in previous versions of TFS (the old default behavior is now called a server workspace.)
With a local workspace, TFS behaves like an edit/merge/commit-type version control system. That is to say that files are not read-only and instead TFS will scan your filesystem to determine what has been changed. You no longer need to check files out from the server and, in fact, the server does not know what you have checked out.
For this reason, you will not be able to see that the other user has checked out the file – this information exists only in his or her local workspace.
If this other user was using a traditional server workspace, the server would have this information and would present it to you.