Once you are finished designing your Website it is important to validate the site for accessibility issues.  There are a variety of validators available for free.  One of the more popular sites is WAVE or WEBaim.  

Web accessibility evaluation tools are software programs or online services that help determine if a Web site meets accessibility guidelines. While Web accessibility evaluation tools can significantly reduce the time and effort to evaluate Web sites, no tool can automatically determine the accessibility of Web sites.

For a comprehensive list of validators please check our the W3C's list of vendors here.