Ask 'em if they are an Andrex. If they take umbrage, it's a person, if not, it's a 'bot.
(I saw a gushingly complimentary post under a blog post which was less than riveting, the other day. Looked like a boilerplate and had a link to their own blog, to boot. Curiously the first blogger wrote a warm thank you, like they hadn't realised it was spam). :o)