Accessibility testing for web sites is a service that can provide much more than the standard point-by-point testing techniques of most automated services.
It provides a more detailed analysis of the content and layout of the page elements, yielding optimization procedures for a variety of circumstances that can be used during the development process of a site, site remodeling, or ongoing evaluation and monitoring of an existing site.
Several software tools which automatically test a web page, or an entire site, for accessibility guideline conformance have been available for some time.
These programs evaluate compliance with a variety of standards, including those of U.S. Section 508, the British Disability Discrimination Act, among others.
The use of these tools is a very valuable step in assuring that a web site meets these standards, but relying solely on their evaluation, neglecting to perform a "manual" check of certain accessibility aspects, can result in circumstances that may exclude some portions of the public from a site.
The Web Accessibility Initiative (WAI) of the World Wide Web Consortium (W3C) has this to say about manual evaluation:
Examine page selection using relevant checkpoints from the Checklist of Checkpoints for Web Content Accessibility Guidelines 1.0.
Note relevant can mean: checkpoints that cannot be evaluated by automatic or semiautomatic tools; checkpoints that actually apply to the site (e.g. if site contains no audio content, skip those); and, as a minimum, those checkpoints that apply to the level of conformance you are evaluating.