A hidden field is not suitable, because palitsya and populated with the appropriate data
well, ask in this hidden field: "You me manage to get the FAQ on the white light cannot be!"))))))))
And as the field is filled - it's a bot.
PS1: There is another option: transparent "proxying" a POST request to a URL mask (s):
1. Verification on variable captcha.
2. If the captcha is not passed - play the Turing test (direct full-page).
3. Passed - pass a POST request to the website and set variable flag that the captcha for this session - passed, and never requested it.