How do I know if its encrypted?

[Eric Hughes]

   Of course, we're not just dealing with text.  So the scheme has got to be
   changed a bit so as to be able to detect unencrypted GIFs, and mu-law
   files, and as yet to be determined unknown files.  

Each of these data formats has it's own regex recognizers available.
Just apply them.

The point, though, is to enforce the presumption that the remailer
operator does not, in fact, look at the traffic in order to understand
the content.  You don't need a completely airtight algorithm in order
to do this.

Eric