There is a large number of evaluation and even repair tools that are designed to help you make your web site accessible. The Evaluation and Repair Working Group of the web Accessibility Initiative has a list of about 122 of these tools on their site.
One of the greatest benefits of using the tools is education. If you try them, you will find reports of dozens of errors or violations even when you thought your page was accessible.
There are many items that cannot be checked by any computer program, like whether or not your web site uses color alone to convey information. A tool can check whether or not images and areas have alt-text and whether or not frames have name and title attributes. But, the tool cannot decide if those are good values, and the tool cannot add meaningful alt or title attributes - that will always be up to you.