There are automated tools, but in my experience they mostly pick up the easy things (missing alt attributes, etc.) The harder bits are focus and notification in SPAs. Read the WCAG docs [1] and the and WAI ARIA docs [2] stick to them. Then test with a screenreader.

[1] http://www.w3.org/TR/WCAG20/

[2] http://www.w3.org/WAI/intro/aria.php