Close your eyes and try using your app.
In other words, try getting into unsighted users' minds. Understand how do they use the web. I'd start from watching some short and sweet videos, e.g.:
Keyboard navigation
https://www.youtube.com/watch?v=N9Q8oF0Lx2M
(!!) Screen reader:
https://www.youtube.com/watch?v=Hp8dAkHQ9O0
https://www.youtube.com/watch?v=q_ATY9gimOM
https://www.youtube.com/watch?v=dEbl5jvLKGQ
https://www.youtube.com/watch?v=7Rs3YpsnfoI
Screen magnification: