How to establish if a string contains a message
It will depend entirely on the specific encryption scheme. There is really no theoretical reason why a ciphertext should not be clearly identifiable as being a ciphertext and not a plaintext. In fact, take any encryption scheme and modify it so that the encryption algorithm works as before, only it appends to the ciphertext the string "THIS IS A CIPHERTEXT!", and modify the decryption algorithm to work as before only ignoring this string. For pretty much any reasonable encryption scheme this modified scheme will be (theoretically speaking) as secure as the original scheme.

Why is this? Because, when we evaluate the security of a scheme we assume that the adversary (i.e., the entity trying to break the scheme) knows what scheme he/she is breaking and is explicitly told what pieces of information are ciphertexts.