Typically, symmetric encryption will in a way authenticate the decrypted data: If data can be decrypted with a given passphrase, it must also have been encrypted with that passphrase. (derp. addendum: not true in the general case if oracle attacks are relevant, and mdc only provides integrity!!)
The flexible way in which GnuPG's cli interface works makes relying on properties unreliable: A call to gpg --decrypt might unexpectedly asymmetrically decrypt, or worse, ask for a passphrase for which decryption fails, and then output a bare literal data packet that comes next in the stream. There are probably other scenarios, but more generally the nested way in which data is handled makes it difficult to reason about this one way or another.