The W3C has extensive information on how to properly evaluation a sites accessibility. Here are the underlining steps to ensure that your evaluation is full-proof:

For a preliminary review, select a page that is representative of the whole site or that most people will see. Try to choose a page that has tabular data, images and scripts. And then:

• Examine this page for alternative text,
• Divs instead of tables for page layout,
• Use the keyboard instead of the mouse for navigation,
• Test with different font-sizes and screen resolutions.

The Firebug and Web Developer extensions in Firefox will make your life easier in accessing the code and disabling images and even resizing your browser size. It might be a good idea to try a screen reader, and not to mention an Web accessibility evaluation tool like AChecker. These will enhance your understanding of the sites limitations and successes.

Another important part of evaluating a site is to get people with disabilities involved in the process. Some may have insights that other users will not.

For a complete procedure of website evaluation you have to go to the W3C – Web Accessibility Initiative page.

Although a little outdated, the WAI also provides a comprehensive checklist of accessibility guidelines (WCAG 1.0) and an useful template for the final accessibility report. They really thought of everything!