The trick with any offline media is to plan some kind of unique value that the person should use to connect with the business, examples would be:
- Custom URL (ex: www.clarityformarketers.com)
- Phone (Ex: 1-888-Marketers)
- Discount Code: use MARKETERS to get 20% of your first call
- SMS a custom CODE to get a discount
- QR Code to receive discount code
etc...

The other then you'll want to do is ask the visitors to your site, or business "How did you hear about us?"

I will say that it's only 40% accurate as most people will think they heard from a friend, but maybe initially got introduced to you via a radio ad, or facebook promotion. It's not accurate, but the easiest of all the options.

Hope that helps - call if you need more
http://clarity.fm/danmartell