I need to make an email opt-in form that pops up only when a visitor is not currently subscribed to our emails. So, the person would get an email from us, click on a link, and then when they get to our blog they would be prompted with a popup to opt in to our emails. The part I am struggling with is figuring out how the popup would "know" whether they are subscribed or not. Any ideas as to how this can be accomplished with Drupal? We use Eloqua for our emails and list management if that helps.