There's a new report available to you under Sales and Marketing on the Reports page. It's the Customer Marketing option:
When you click it, you'll see a list of customers created within the report time frame, grouped by the Source field. That's the configurable "Where did you hear about us?" option on the Contact Form. (You can also add it directly to a customer in the Additional Contact Information section.)
This will tell you at a glance how your customers are finding you, and which of your promotional efforts are paying off the most. Any customers for which the source is unknown will show up under Unspecified at the bottom of the list.