To determine whether a software application is accessible, the application should be evaluated according to a set of guidelines or standards that defines software accessibility.
The only legal standard is Section 1194.21 ("Software applications and operating systems") of Electronic and Information Technology Accessibility Standards . These standards were developed by the Access Board as required by Section 508 of the Rehabilitation Act Amendments of 1998. There are twelve standards, and the Access Board provides a detailed explanation of each in its Guide to the Standards .
Following publication of the Access Board standards, the Information Technology Industry Council (ITIC) partnered with the U.S. General Services Administration (GSA) to create a Section 508 compliance checklist called the Voluntary Product Accessibility Template™  (VPAT™). Many software companies have completed VPATs and have made them available on their company websites. GSA has also created a central database called Buy Accessible , where many vendors have posted their VPATs. The database serves to assist purchasers in making informed decisions about products' accessibility. To date, there is no comprehensive third-party evaluation of software; so purchasers must rely on vendor self-representation in the VPATs, coupled with their own understanding of product features and accessibility issues.
IBM separately maintains the IBM Software Accessibility Checklist . This checklist includes twenty checkpoints in seven categories, and each of the checkpoints links to specific techniques documents for software developers.
A few postsecondary educational entities have developed their own checklists, guidelines, and policies for ensuring that accessibility is considered when software is purchased. Examples include MIT , Oregon State University , and the University of Minnesota .
Additional information specifically for software developers is available in the AccessIT Knowledge Base article How do I develop accessible educational software? 
-  Electronic and Information Technology Accessibility Standards
-  Guide to the Standards
-  Voluntary Product Accessibility Template™
-  Buy Accessible
-  IBM Software Accessibility Checklist
-  MIT
-  Oregon State University
-  University of Minnesota
-  How do I develop accessible educational software?