I know of one way and there may be others…
1. Go to the machine that is sharing the document and right click the my computer Icon and go to manage. (or connect and manage the computer sharing the file from your machine.)
2. Click on shared Folders
3. Click on Open Files
4. Find the file in question and you should be able to see the user name of the person that has the file open You can close that connection to the file from here if needed.
I have seen this many times and this always works. Good luck.