Along with Google Webmaster Tools, Google Analytics is an indispensable tool for measuring your web traffic trends. You should be tracking your number of unique visitors on at least a monthly basis. It is also helpful to check the referring traffic which will give you insight into where most of your traffic comes from. Finally, advanced segments allow you to divide the paid traffic and SEO traffic, as well as allowing segmentation of several different traffic sources into separate channels.

The best part about Google Analytics is that it is free.

Make sure that you have Google Analytics correctly configured for your website with the SEO Checklist