For privacy reasons, I want to prevent my users from posting unencrypted files to my ftp site.

It is company policy that all data exchanges of sensitive data be encrypted with PGP.

I'd like to setup a program to monitor the ftp folders and whenever a new file is placed there, verify that it is in fact encrypted.

I can't just rely on the file extension because in some cases, our trading partners require a specific filename that doesn't have a .PGP on the end.

Is there a library or another method I can use to verify that a given file is encrypted?

I'm using C# and .NET on a windows platform.