If you are using Firefox or IE, you can use google toolbar. Install it and activate the google PR option from options of toolbar. When you visit any site, you will see a little gray bar. If that site have page rank is will be filled with green color. When you put your mouse over it, it will show you how much PR the site has. I hope this will be helpful to you.
Google just announced that they won´t support Google toolbar for Firefox anymore. You can still use it for IE. I recommend you to install other SEO addons that also include PR scores, like seobook, or seoquake.