Assert the page is WordPress's refusal for a missing capability: the "Sorry, you are not allowed to access this page." screen. It is what a role sees on an administration screen it may not use.
Examples
Then I should see the access denied screen
And we should see the access denied screen
Then should see the access denied screen
Then I should not see the access denied screen
But we should not see the access denied screen
Pattern
Then(/^(I |we )*should( not)* see the access denied screen$/, …)
Part of @webship-js/wordpress-steps (github.com/webship/wordpress-steps), a step pack for Webship-js 2.0.x.