Through the effective use of Google Analytics you can track what the traffic to your site is doing. The goal of SEO is not just to drive more traffic, but to drive better traffic. You can track if a user completed an online form for example, or analyze how long they remain on your site and then be able to determine if there is a correlation between time spent and becoming a customer. Additional information is found in the full article at another link.
Read more: How to track conversions like a pro