Is there a way to track where traffic is coming from when looking at the list of contacts that filled out a form?
We use URL Parameters to track where the web traffic is coming from. This hidden field in the form also determines which SFDC campaign the individual gets added to from form fills - which is one way we measure success. Here's the link to the hidden field info: Set a Hidden Form Field Value - Marketo Docs - Product Docs You can create your parameters in the URL builder to use in your campaigns.
Emily Dick's idea is great! Using URL parameters are awesome if you're pushing a lot from your website. You can pull how many people visit the form from an email through the clicked link reports, but it seems like you're more concerned with external visitors. You could also incorporate Google Analytics and some tracking if you're embedding your forms directly onto landing pages and not necessarily using landing pages all the time.
I don't have a step by step guide but I can tell you how we do it at my company. We use parameters in a couple ways:
Edit your form -> select the field -> change type to Hidden -> edit the field -> Get Value From select "URL Parameter"
The Parameter Name you enter is the name you gave to the field in your URL. We use Google's UTM names. For example, utm_source = Lead Source
Marketo will automatically pull in the value (such as Content Syndication) into the field from the URL.
Hope that helps!
Here's a guide I wrote that shows how to use URL Parameters for lead source tracking. Hope it helps!