Useful tools
There are many different tools that can be used to help check your app's accessibility. Some can be run in a browser, while others require use in an emulator or on a physical device:
Accessibility Inspector, available on OS X and iOS
VoiceOver, available on OS X and iOS
TalkBack, available on Android
Accessibility DevTools for Chrome at https://chrome.google.com/webstore/detail/accessibility-developer-t/fpkknkljclfencbdbgkenhalefipecmb
Accessibility panel in Safari/WebKit (right-click on An Element and then on Inspect Element. It will show the node details in the sidebar. Then. you have to expand the Accessibility panel)
Tip
Although the earlier mentioned tools are incredibly useful to test your app's accessibility, your users will be the best source of information regarding your app's usability. Always test your app with your users to see how the accessibility fares in real life scenarios.