How to tell if a file is open by another process?

[Jason W May]

I am trying to figure out how to determine if an existing file is
already open by another process (over which I have no control).
In this case, the other process is the ftp daemon: I am watching
a particular directory for files to appear, and I want to be able
to tell if an incoming file has been completely transferred.
Is there a way to do this, besides watching to see if the file size
changes?

Thanks,

Jason