Every spot-the-AI game I've seen uses modern text as its control, so it's hard to prove it wasn't entirely written by AI.

The human paragraphs here are verbatim from public domain works published between 1660 and 1922 (Pepys, Gilbert White, Twain, Jerome, the Grossmiths, Wodehouse), so they predate the clankers by a century; the other half is Opus 5 continuing in the same voice. There are 10 rounds, I managed to get a ~60% win rate.