Tim is almost correct... You'll notice the form ID he mentioned uses hyphens (-), whereas the form IDs required by Captcha and Antibot use underscores (_).
The correct way to find a form ID for use with anti-spam modules is to look for the hidden input element in the form with name 'form_id'. The value of this element is what you want:
So in this case (for the comment form at the bottom of this Forum page) the form ID is: comment_node_forum_topic_form
In this case this form ID is different to the ID attribute on the form element itself, but generally they are the same (save for using hyphens vs. underscores).