1. Log on to your Google Adsense account. There are four tabs at the top of the page. The defaulted one (the one you're on) is "Reports."
2. Click the "Adsense Setup" tab.
3. Choose the type of Adsense ad you'd like to add to your website. Continue through the ad-creating interface as usual.
4. When you reach the page that allows you to add channels, stop. Here, click the "Add New Channel" text link. Name the channel something that you will readily identify as pertaining to this particular site. For instance, if you own abc.com and xyz.com, and this add is for the latter site, name your channel xyz.com.
5. Next time you log in to your account to check your Google Adsense statistics, click on the "Top Channels" text link on the Reports page. This will give you a breakdown of your number of page views, number of clicks, CTR (click through rate), and earnings by channel. This will enable you to determine which site is earning more money for you.