Use two passes. In the first pass over the directory, make a list of all files in the directory and their size. Ten minutes later, if that size hasn't changed, move them to wherever it is you want.

The logic, in more detail, might look like this:

• Read the directory. For all new items, add them to your in-process log.
• For all pre-existing entries, determine if their size changed since they were logged ten minutes ago.
• If size didn't change, they're ok to process. Do whatever processing you need to do, move them to wherever they need to go, and remove their entry from the log (or mark it processed).
• If size did change, write new size to the log so that file can be revisited when the script runs again in ten minutes.

This doesn't guarantee the tar's aren't corrupt, but it does at least give them time to finish being sent.

Update:

Another strategy might be to attempt to get a flock on the tar. I'm suggesting this under the assumption that the FTP server locks the file while it's still being written to. If that's the case, you won't be able to get a flock on the file while it's being uploaded. And that will tell you whether or not it's safe to process it.