How do you test accessibility with users when you only have prototypes?

I want to do some accessibility user testing, but don't have the built product yet. Much like usability testing i want to test as early as possible and as frequently as possible.

Will a prototype, if high fidelity, work for accessibility testing in a lab with users?

I am kinda assuming that it will work for a lot of things but not others.

I'll likely be building it in Axure or Proto.io (please feel free to suggest another tool if better for testing for accessibility) so am assuming that i won't be able to test for the use of screen readers and keyboard driven navigation with these prototyping tools.

What are peoples experiences advice?