Perhaps some notion of entropy would be useful here? Normal information theoretic definitions of entropy usually rely on "bits of entropy". I.e. if you are given information in the form, "Should I compare %s to a summer's day" then how many new bits of information are we actually getting? In this case, it is unbounded, because what we are calling `name` can be arbitrarily long.

But what about if we define entropy in terms of something like parts of speech? Then, the computer can only change one part of speech (the noun phrase to which we compare a summer's day) whilst maintaining the context of the sentence, so in terms of that metric, entropy is low.

With this parts of speech entropy metric in hand maybe we could declare the output of a computer to be "written by a computer" if the entropy in its sentences approaches that of a human writing about the same topic.

Disclaimer: I spent about as much time thinking about this as you did just reading it, so I don't expect this is in any way a fully formed or implementable idea, just food for thought.