Prefixing data before encryption for passphrase confirmation?
There is no risk if the attacker knows part of the plaintext. This doesn't help to recover the rest of the plaintext. Even if the attacker knows all the bits but one, with any decent encryption algorithm, the attacker cannot tell what the last bit is from the ciphertext.

After all, it happens all the time with known data formats. For example, if you download a web page over HTTPS, the attacker knows that the page begins with &lt;html&gt;&lt;head&gt; or some such.

Note that even if you find the expected prefix, this doesn't guarantee that the rest of the data isn't corrupted: encryption guarantees confidentiality, but not integrity. You need either to use an authenticated encryption mode (which Crypto-js doesn't provide) or to also transmit a MAC of the ciphertext (encrypt-then-MAC).