There are excellent validators such as Bobby,
which Richard mentioned. You can enter a single page's URL and see if it meets ADA or the government's 508 guidelines. Bobby will give you the output of what is wrong and suspect.
There are software programs to test entire sites (one was offered on Bobby and I think later offered for sale) and to help as the site is authored.
Dreamweaver has a plug-in, but I don't know about FrontPage, Netscape or other programs. These help prevent the author from adding "problem" code. Some standalone programs are also available to test a site all at one time. Reports are generated specifying what is wrong and where.