How to know if any website is using Google Website optimizer for A/B testing? By viewing a source code. For what to look into view source which confirms that site is using Google Website optimizer
Look for the _utmx cookie for the site in question.
You can use Google Analytics with Google Website Optimizer (GWO), which is a tool that helps determine the most effective design for your site. When a website optimizer script executes on your page, a _utmx cookie is written to the browser and its value is sent to Google Analytics. See the Website Optimizer Help Center for more information.