You can use the "Original Referrer" filter to see if there were specific UTM parameters in the URL when the lead first visited your page. Typically you would use "Contains [UTM Parameter]=[Value]" for specific values.
Seems like a lead could visit you as an anonymous lead and never fill out a landing page. In that case, the anonymous lead has important information that may never make it into your MArketo custom field. If you later merge the anonymous lead and the known lead, how can you get that information into the field? Does anyone routinely do this?