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 Knowledge Base article How do I develop accessible educational software?