Tracking The Source of Optins...

You could use tracking202 or even your own method of tracking - just embed a tracking code in the url when you link back to your site, capture it on the site, and then embed it in Aweber's code in the value section:
<input type="hidden" name="meta_adtracking" value="">

If your page consists of a single squeeze page without any other links on the page, google analytics can help you keep track of your optins very easily.

Once you login to your main dashboard, on the left hand side, you'll see the navigation menu:
Go to Traffic Sources -> Referring Sites.

There, it will list all of your incoming traffic sources. On the very far right hand side, it will list a value called the "Bounce Rate." This tells you what percentage of the traffic from this specific source left without going onto any other part of your website.

The second column shows the number of visits. If you multiply the number of visits by (1-[(bounce rate)/100]), it will tell you how many of your visitors opted in from that specific source.

A little bit of work and a little bit of math, but generally precise calculations. You could also just look at the numbers and get a general feel for what traffic source has the lowest bounce rate and try to get more traffic from that source.

Hope this helps!