There are a couple of startups already looking at tracking customer behaviour in the real world based on MAC address[1] but hands down the best way of tracking the success of a marketing campaign is with coupons. If you're running Google ads for a bricks and mortar business, don't send people to your home page, send them to a landing page which has a special offer on it, or some other identifiable thing so that when people come in and ask about the XYZ you know where they came from.

[1]http://www.fastcompany.com/3004781/google-analytics-real-lif...