The simple question is how do you sync a Windows file system to an FTP destination periodically while never checking the destination for change unless something bad happens and not relying on file size or date/time changes. Too many other differences exist when using date/time to determine change (NTS alters seconds, daylight savings time, etc.) and file size is not as reliable because of blocking factors, etc.
The first issue is which FTP server software out there supports checksums or hashes using MD5 (MD5 is pretty quick compared to SHA-1, but whatever works).
Second issue is which copy program can generate hashes from a directory listing, store them in a database, send over anything that changes or is new, and never check the destination FTP server unless oddities happen, possibly having a server side utility to generate the database if files already exist.