When uploading a large file, if you want to work within a directory full of files, you will find that it is quite difficult to ignore the ones that are being processed.

If you’re on the actual server itself, you can use ftpwho. http://linux.die.net/man/1/ftpwho
This is shows the current active processlist of files being uploaded by FTP.

Alternatively, and I found this by far the easiest way to distinguish between a file currently uploading and a file that is already uploaded. In comes “HiddenStores”:
http://www.proftpd.org/docs/directives/linked/config_ref_HiddenStores.html

This will mark a file with a ‘.in.’ at the beginning of the file name, so that you can easily identify a file that is being worked on at that point in time.