FTP does not lock files when it is still writing to them (I've run into this issue already where a 500 Mbyte file was being uploaded and our process picked it up and moved it on to another location before it had completed the upload). Our solution in that case was to have to sender create a "lock" file once they had completed transmitting the file to us. The lock file is usually very small (0-byte preferrably) that was we look for the lock file and then perform the actions we need on the other file. It might not be the cleanest solution, but it seems to work.
in reply to how to tell if a file is still being modified
"Ex libris un peut de tout"