How do I know if its encrypted?

[Eric Hughes]

   From: Ben.Goren at asu.edu

   That of the data haven operator being able to deny knowledge of the
   contents of files people send him. He'll only return files that, when
   operated on by a strong cryptographic algorithm, make sense. 

This idea doesn't work for the purpose intended.  I'll upload straight
ASCII.  When you ask for an decryption key, I'll make one up randomly,
apply the decryption algorithm to the flat text, and send that back to
you as a confirmation.

The real question is "Makes sense to whom?".  You can't enforce a
requirement of encryption, but you can make sure that you can't make
sense of most of it.

   As best I can tell, none of the previous suggestions guarantees that the
   file is unreadable.

You don't need a guarantee of unreadability.  What is needed is a
presumption that files were not read.  If they are unreadable, then
they weren't read, but there are other ways of creating that
assurance.

Eric