It's hard to ever get this resolved too granularly.
Someone might see 10 of your ads, and every ad mentions your URL, and it's the worlds easiest URL, something like travel.com. Yet a good fraction of your users will STILL go to Google, or Yahoo, or whatever search engine was setup by default in their browser and type in "travel.com" and then click the top link to your site. So, do you attribute that hit to the search engine? The radio ad? The email(s) you sent them?
As others have mentioned, the use of specific landing pages will help filter things somewhat. Also, engagements on social media sites can help you resolve where interest is originating from.
But, at the end of the day, marketing is a little bit like scattering wildflower seeds in the wind and hoping that enough of them take root to grow into something pleasing.