Bruce Schneier has a nice little primer on how to recognize plaintext.
http://www.schneier.com/crypto-gram-9812.html#plaintext
That's interesting. But wouldn't the overhead of testing each decrypted message slow down brute forcing considerably? It's one thing to try a billion keys a second, and it's quite another to look at each decryption and decide if it's any good or not.
If the encryptor knew the plaintext recognition algorithm, he could also take steps to dodge recognition:
LikeHereIsMyPlainTextMessage.
If they're looking for spaces they're out of luck.