The easiest method is to use a separate form for each page, and either Tag the contact accordingly for the page or look at the form's statistics.
You can also use a 3rd party tool like Google Analytics to determine questions like "of the people that visit page X (opt-in page), how many people visit page Y (thank-you page)."
Remember that conversion tracking is built into ONTRApages, and you can include ONTRApages in your WordPress site using the ONTRApages Plugin.
For non-WordPress sites you can include the ONTRAPORT tracking script.
You can get very fancy in ONTRAPORT. You can create Dashboard Metrics for a visit to that ONTRApage or tracked web page where the contact has a "Date Added" date within the last 7 days. Or use Marketing Trackers to track every link you create.
But knowing what is significant to track is as important as tracking itself. We frequently see clients get too caught up in the most recent tracking fad and miss out on what is really important: growing their business.
If you are feeling overwhelmed it might be best to get some help setting this up the first couple of times - there are lots of ONTRAPORT Certified Consultants that can help with that.