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)