There are many tools that can assist in developing accessible web pages.
WAI Tool List WAI's list of tools.
Techniques for WCAG 2.0 Good list of 100's of solutions to Accessibility issues.
The results from most accessibility tools should be subject to manual review.
Cynthia Says Online testing of a webpage for accessibility
Wave 4.0 Online testing of a webpage for accessibility
Test at least 6 of your web pages.
Correct any errors that are reported.
The W3C WCAG have a formula that calculates the contrast between foreground and background colours.
Contrast Tool Test out a few colour combinations.
The Firefox extension Fangs, allows sighted users to read a web page the same way that the JAWS screen reader would speak it.
It provides an invaluable insight into how your web pages are experienced by a blind user.
Download the Fangs extension and use it to check ten web pages.
Make a list of the pages and rate them in order of most accessible to least
accessible, add comments where appropriate.
Chris Pederick's excellent Firefox 'Web Developers Toolbar' has multiple tools for identifying accessibility issues.
It should be used in conjunction with this articleFirefox Accessibility to help improve the accessibility of your web pages.