If you are on linux it is quite easy.  During the sync process you can do:


`find $HOME/dropbox_sync_dir -type f -exec dropbox.py file status -a '{}' \; | grep 'syncing'` 


**Explaining details:**

 - `find <path> -type f -exec dropbox.py ... '{}' \;`

Runs `dropbox.py`  on every file or below the current directory.  

> Notice that the braces are enclosed in single quote marks to protect
> them from interpretation as shell script punctuation. The semicolon is
> similarly protected by the use of a backslash, though single quotes
> could have been used in that case also.

More on [man find][1] pages

 - `dropbox.py file status -a` from the [dropbox linux client manual][2]:

        dropbox file status  [-l] [-a] [FILE]…
    
        Prints the current status of each FILE.
    
          Options:
    
           -l or --list prints out information in a format similar to the native Linux command ls.
           -a or --all do not ignore entries starting with "."
    
       Aliases include stat.

 - `| grep 'syncing'`

Searches for the 'syncing' string.

Note: that the .py end should not be mandatory there should be an alias so writing just `dropbox` should be enough.

**First Edit** - re-edit due to Paul's comment about dropboxd (daemon) vs. dropbox.py.

The `dropbox.py` should be debugged via `pdb` (the python debugger) for more at [python debugger][3].

Finally after the Paul's comments I understood where is the core of the issue.  He wants to see the *currently accessed file* or even better *the one being transferred* by the dropbox sync client.

 - You could find all files used by the dropbox client currently used:

        ls -thal /proc/`pgrep dropbox`/fd | egrep -v 'socket:|pipe:|anon_inode|/dev/'

 - If you can not see anything that means that something is probably
   wrong with the client.  I would recommend performing [strace][4].

        To monitor your dropboxd script with output save into output.txt file:
        `strace -o output.txt dropboxd`

        To monitor only system calls you could enter:
        `strace -e open dropboxd`

 - If you want to go even deeper you could attach an *debugger* e.g.
   `gdb` to your `strace` session.

  If you are unfamiliar with this process you can check,for example, this nice [post][5] about unix `strace` and `gdb`.  

**Just a quick help:**

When you start your trace with: `strace dropboxd` (running pid would be 501)
Then you attach to it the following way:

    $ gdb --quiet
    (gdb) attach 501


 - If everything else fails you can use other ways to sync your dropbox:

For example, use a different cli client like [dbxcli][6].

**Second Edit** 

It occurred to me that it should be possible to check what is transferred even via network monitoring tool like `wireshark`.  

Since the dropbox client is using SSL/TSL (or AES) [The dropbox security architecture][7] and you should have the key to check.  

For details how to use `wireshare` you could check `https://wiki.wireshark.org/SSL` or adjust this example to your case [Analyze SSL/TLS Client Hello Traffic][8].  In you have dropbox certificate you could import it into wireshark to see what is happening. (For dropbox security details you can check [Dropbox security whitepaper][9].

**Third Edit** - details about the SSL/TSL decryption.

In the second edit section I suggested that you can view the SSL traffic via wireshark.  Since Paul has thought that I'm suggesting that you should actually hack the `DH key exchange`, which is with current knowledge not possible, I would like to rectify it.

What I'm trying to suggest is that if you are using [dropbox business RSA][10] and RSA authentication, you could try to use the `dropboxd with RSA key` (the daemon dropboxd is closed source, I can not check the source code if the functionality is there and probably the best solution is to ask dropbox.com directly).

If that is possible, only then import the `RSA key` to [wireshark][11] to see what is currently being transferred.


  [1]: http://%20https://linux.die.net/man/1/find
  [2]: https://www.dropbox.com/help/desktop-web/linux-commands
  [3]: https://docs.python.org/2/library/pdb.html
  [4]: http://www.thegeekstuff.com/2011/11/strace-examples
  [5]: https://www.ibm.com/developerworks/aix/library/au-unix-strace.html
  [6]: https://github.com/dropbox/dbxcli
  [7]: https://www.dropbox.com/business/trust/security/architecture
  [8]: https://en.wikiversity.org/wiki/Wireshark/HTTPS#Activity_4_-_Analyze_SSL.2FTLS_Client_Hello_Traffic
  [9]: https://cfl.dropboxstatic.com/static/business/resources/dfb_security_whitepaper-vfllunodj.pdf
  [10]: https://www.dropbox.com/business/app-integrations/RSA
  [11]: https://support.citrix.com/article/CTX116557