Is it possible to decide what is a gpg file?

[Robert J. Hansen]

Melikamp The Medley wrote:
> I mean, is there a reliable way to tell that something is _not_ an
> encrypted file?

If you mean, "a reliable way to tell that something is not an
OpenPGP-encrypted file," then yes: check the OpenPGP header at the
beginning of the message.

If you mean, "a reliable way to tell that something is not an encrypted
file, period," then no, not really.

There are a lot of qualifiers on the "no, not really."  A lot of Ph.D.
theses have been written on this subject: it ties into some really deep
areas of theoretical computer science.  If you want to learn more about
the qualifiers, I'd suggest reading up on algorithmic randomness and
Kolmogorov-Chaitin complexity.  It won't be easy reading, but speaking
personally, I find this stuff fascinating.