Testing accessibility can be divided into platform specific testing and API testing.

Tools for platform specific

Windows:

Use UIA native testing tools such as UIA Verify and  Accessibility Checker.

http://msdn.microsoft.com/en-us/library/windows/desktop/dd373661(v=vs.85).aspx

Mac:

Use Accessibility Inspector and Verifier. See weblink below:  

http://developer.apple.com/library/mac/#documentation/Accessibility/Conceptual/AccessibilityMacOSX/OSXAXTesting/OSXAXTestingApps.html

FX Ally testing:

Expand ScenicView for ally hierarchy and property values.