Classify ciphertext vs. plaintext
Simply counting the frequency of characters ought to easily distinguish between English language and ciphertext, because they're so obviously different.

You can just count the frequency of characters in a big corpus of English, and a big corpus of ciphertext, and apply a chi-squared test to each to figure out which one matches the counts in a new chunk of text.

Or if you can assume ciphertext has a roughly uniform distribution over characters, that alone lets you construct a good test for whether new text is unlikely to be ciphertext.

I did a short blog post on something similar. https://blog.cloudera.com/blog/2016/09/solving-real-life-mysteries-with-big-data-and-apache-spark/