While you can use automated validators to measure whether you meet section 508 and WCAG accessibility guidelines, often making the interface usable for screen readers requires more. The validators simply tell you if you have violated one of the standard rules. It does not tell you if the user can make sense of whatever it is that has been written.
Also it should be noted that while section 508 seems mostly focused on users who use screen readers, there are a number of other disabilities that are covered as well including cognitive and other various forms of visual impairments. To truly satisfy users who need section 508 assistance you need to test with users who represent those types of users. Truthfully with many eCommerce sites failing at this, sadly the bar is set pretty low to meet their needs.
If your site has a global presence then you really need to meet the WCAG Level A or priority 1 requirements at the least. As WCAG is the standard set forth by the W3C it is best to follow what is set as a standard by this group. A great article on the differences between the two can be found here;
Other Accessibility resources are here;