Pretty much any question that requires actual natural language understanding would presumably instantly reveal a bot today, like 'Is white darker than black?'.
It's not that simple. NLP has gotten good enough that there actually are systems which can answer questions like that. IBM's Watson can quickly sort through a massive number of documents for an answer. There are projects that have built massive databases of common sense knowledge like Cyc, which could possibly answer that question. And then large databases of human responses like cleverbot which probably already has been asked that at least once and can find it in it's database.
