making sure a file isn't being written/copied before moving

So, I'm (still) working on some scripts to rename and reorganize my image 
files.
Right now, I have a script on my laptop that pulls images off the CF card 
and stores them locally. I have another script that rsync's the images to 
my server at home whenever there's a connection available. These scripts 
don't step on each other because they look at the process list for 
instances of rsync.

But I need to write two more scripts that take the image files from an 
incoming directory, rename them, and drop them into a directory for me to 
work on them; then once I've done whatever I'm going to do (cull, keyword, 
etc), then I put them into a directory for archiving. The images are going 
to be pulled from that directory, put in an archive directory, and have 
the immutable extended attribute (xattr) set.

My problem/issue is that I don't want to do anything with a file that is 
in the process of being moved into one of these directories. I have to be 
sure that the file is not still being moved/copied. Now, these directories 
are all on the same filesystem, so it *should* be an atomic change by the 
filesystem (technically a rename of the file from one directory/file name 
to another). But I want to be sure--plus I may be dropping files into the 
incoming directory from elsewhere from time to time. (I plan on going 
through my backlog of old untagged files eventually.)

Can someone suggest an easy (or at least reliable)  way to make sure that 
any file I'm about to modify isn't being touched by another program?

Paul