Hey Matt! Great question. There are two main ways to track attribution for digital brand/awareness campaigns:
If you're looking for a tool to help, quite a lot of Marketo customers are using Bizible to measure brand attribution as it includes referring domains, UTM tracking, and view-through conversion.
One little trick that I picked up when developing a methodology for how to attribute "first touch" to events which happen prior to the lead's being created with Oktopost is that you should have a smart list "acquisition exceptions" which is defined as something like If UTM campaign is not empty and then use that for any "on site" conversion events so that if they fill out a form on the site it WON'T let the form be the acquisition program.
After that, your "pre-conversion" acquisition programs need to listen for new leads withe the correct "UTM campaign" value. Once you do that you'll successfully be able to differentiate between the sorts of activities you're doing on to drive people to your site, and the "organic" results of your site.
It's a bit of work to implement, but the results just look sexy!