There is no way you can force checkins of files in other workspaces. Files checked out are only present on the local drive where the workspace is located, so if for instance the machine is turned off, there is no way to get to the files.
But you can identify files that are checked out, and notify the users. Using tf.exe you should be able to get a list of all files checked out for all users like this:
tf status /user:*
You can limit the scope by suplying a path and /r to get all items under.
tf status /user:* $/TeamProject/Project/ /r