Web accessibility is defined by guidelines and standards such as the World Wide Web Consortium's Web Content Accessibility Guidelines , and the Electronic and Information Technology Accessibility Standards , developed by the federal Access Board as required by 1998 Amendments to Section 508 of the Rehabilitation Act. For more detail about these guidelines and standards and how they relate to one another, consult the AccessIT Knowledge Base article What is the difference between the W3C� Guidelines and the Section 508 standards for web accessibility? 
In order to determine whether particular websites are accessible, there is no substitute for developing an understanding of web accessibility principles, including the guidelines and standards. However, there is a growing body of tools to assist users in conducting both manual and automatic web accessibility assessments.
Automated web accessibility evaluation and repair software can play a role, particularly in assessing large sites. Most of the products available in this category include the ability to spider or crawl websites, following links to other internal pages and including those linked pages in their assessment. Given this ability, these software tools are able to evaluate very large quantities of websites, whereas conducting a similarly large manual evaluation is not practical. Additional information about these software tools is available in the AccessIT Knowledge Base article What web accessibility evaluation and repair tools are available? 
While automated tools are excellent for establishing benchmarks and tracking progress throughout a school or district, they have significant limitations. Most notably, only a small fraction of web accessibility guidelines can be automatically validated. All others require some level of manual inspection. A number of checklists have been developed to assist with manual web accessibility evaluations. For example, the U.S. Department of Justice developed a Section 508 Self-Evaluation  questionnaire to help federal agencies as they seek to comply with Section 508 standards developed by the Access Board. The 508 standards include sixteen standards that focus specifically on "web-based Intranet and Internet information and applications," and the Access Board has also developed support materials to help clarify the standards. The web section of its Guide to the Standards  provides an excellent tutorial, including specific examples and common questions partaining to each of the standards.
WebAIM©, a project funded by the U.S. Department of Education, has also developed a Section 508 Checklist , which includes two parts—Part 1: for HTML and Part 2: for scripts, plug-ins, Java™, etc.
-  Web Content Accessibility Guidelines
-  Electronic and Information Technology Accessibility Standards
-  What is the difference between the W3C� Guidelines and the Section 508 standards for web accessibility?
-  What web accessibility evaluation and repair tools are available?
-  Section 508 Self-Evaluation
-  web section of its Guide to the Standards
-  Section 508 Checklist