>Simple inspection of the DOM and CSS ought to be enough to determine whether the field is visible and nearby the other fields.
Creating a set of heuristic to detect an invisible (to the user) form would be difficult.
My intention was to pose a challenge to prove that honeypot fields do work, and to distinguish between good and bad implementations (e.g., {display: none}).
If you actually rtfm, you'd realize that the purpose of the honeypot textfields is to create them so that they look like they are visible, to bots, but when rendered on a page they would be invisible to a human (for example: textfield may be under another element, positioned off the viewport, or background set to match the page background). Building a bot to detect all those would be incredibly difficulty. To start, the bot itself would have to actually parse the CSS/HTML/JS and render the page, and then run heuristics to figure out which fields should be ignored.