Is it possible to decide what is a gpg file?

[Melikamp The Medley]

Hi everyone!

Sorry if you get two of these, I screwed up while subscribing
to the list.

I have a question relating to the symmetric encryption. If I do

gpg -c foo-file

and enter a passphrase, I get an encrypted foo-file.gpg.
Is there a way to tell that it is an encrypted file just by
looking at the contents? I mean, is there a reliable way to
tell that something is _not_ an encrypted file?