Indeed, unless the traffic/file is explicitly labeled as encrypted (protocol, file header etc.) I think there's no way to tell if a particular piece of data is encrypted or not.
More generally, the incredible stupidity in those calls for encryption bans is that the "bad guys" will abide to the law. There's a dark market for botnets etc., so why not for custom stealth encryption tools? I wouldn't be surprised if it already exists.
That's a very good point; even if it's possible to detect steganography of encrypted messages it's probably expensive to do so, especially in the torrent of digital traffic online.
I guess it will only impact law abiding citizens then.
More generally, the incredible stupidity in those calls for encryption bans is that the "bad guys" will abide to the law. There's a dark market for botnets etc., so why not for custom stealth encryption tools? I wouldn't be surprised if it already exists.