When you've created and validated the entire XHTML file you should perform an accessibility check. Remove all style sheets and JavaScript from the XHTML and carefully look at this unstyled page. Is the content ordered logically? Is the navigation clear and usable?
If content and navigation are usable, you've passed the first test. You should perform many more checks to ensure perfect accessibility, but this rough test helps you catch the most important and serious issues.
Although Bobby offers an accessibility validation service, its results are unclear and sometimes confusing and accessibility specialist Joe Clark has criticized its methodology. Besides, many accessibility features, like using the simplest possible language, cannot be validated by a computer; they need human eyes.
To dive deeper into accessibility issues, read the official W3C Web Content Accessibility Guidelines. If you want some practical examples and tips, the excellent accessify.com website is your best bet.