The main thing to realise is that there is a great diversity of bots, but each is limited in domain, so your best tactic would be to try very varied questions.
The average chatbot or spambot won’t be able to handle deliberately abusive misspelling, but a commercial chatbot will, because they can afford the resources involved in spellchecking. This is still the most effective tactic though.
Second-best is asking word games, like “How many vowels in the xth word of your last response?”. Because there is no reasonable purpose for word games and there are many variations possible, few bots are made to answer them.
However, with chatbots I generally just look at how often they return generic all-purpose replies like “yes, I do” (do what?), or if they keep referring to what I said as “it” for several bouts without topic-relevant details. A lot of bots don’t handle pronouns because that requires resource-heavy parsing.
As for troll bots, whenever I found insinuative vile spewing posts they typically turned out to be from real Americans. Bots’ subject matter seemed more focused on controverse than provocation. The following features are common:
- Outlandishly foreign-seeming usernames with awkward and long combinations of syllables (generated)
- Repetitive subjects and frequent posting history (only equalled by some obsessive autistics)
- Lack of interactive activities (no replies, no following, followers, interests or subscriptions)
- Bad grammar (similar to Indian and Russian, with verbs and prepositions out of order)
- Bare-bones profile information is a bonus
For the time being, troll bots are best detectable through meta-data and posting history. Since a bot’s purpose is to spam, it should post as often as it can, without for instance the inescapable 8-hour period of inactivity known as sleep. I heard there’s a famous Russian bot in America that starts anger-Tweeting at 3AM in the morning every day. That’s definitely not human.