Checking to see if the field is visible is an extremely difficult task. It should be hidden by CSS, so you would need to download all external style sheets and

Checking to see if the field is visible is an extremely difficult task. It should be hidden by CSS, so you would need to download all external style sheets and parse all inline and embedded styles. Same goes if the field is hidden by JavaScript, only it becomes much harder then. The only problem with this technique are dumb auto form-fillers.