How to determine if a file transfer is complete

We have one large file that is sent to us monthly, that goes to the IFS.
We then have an automated process to decrypt the file for internal
processing.  
On this file only, the decryption process fails.  All I have to do is to
move the file from an archived folder back to the original folder and
our automated processing will decrypt the file successfully.
We think that this is a timing issue as this is the largest file we
receive.  Our automated process runs every sixty seconds to check for
files to process.  We can not increase the time between when this
automated job runs.  All other files that we decrypt using these
procedures process successfully the first time.
Is there an API or a trick that we can use to verify that the file
transfer is complete to the IFS before we try to decrypt the file?  



Bruce Barrett
PMI Mortgage Insurance Co.
3003 Oak Road
Walnut Creek, CA 94597
925-658-6152
Fax 925-658-6412
Cell 510-520-4263
bruce.barrett@xxxxxxxxxxxx