How do you track referring URLs in Marketo?

The simple answer is to use the Referrer descriptor on the Visits Web Page trigger.  There may be another way.  

There is also the Original Referrer field that captures the referring URL on the first recorded visit.

What are you trying to do?

I am trying to better track how leads are coming in, since you can't track the source of event registrations if they are already in our database (and therefore already have a lead source established).