Although I don't believe computers are smart enough to detect human pedophiles, I bet it wouldn't be too hard to develop a system that could reliably discern between chatbots and humans. (Cameron Marlow suggested to me in an email that it would be valuable for CS students to consider how they're able to tell the difference themselves -- what cues they notice and how they do their classification.)
With the current state of the natural language processing art, I think the classification problem is pretty trivial. A computer system could be fed samples of transcripts with human and robot participants, and little more than a statistical analysis would be necessary to categorize the differences. After all, computers can already tell the difference between male and female authors with 80% accuracy.
Just for fun, go try the gender genie with some text written by you (or your favorite author). Pasting in the front page of my blog reveals that I'm male!
(NOTE: The genie works best on texts of more than 500 words.)
Female Score: 10987
Male Score: 17405
The Gender Genie thinks the author of this passage is: male!