Assert one of the site's built-in forms is, or is not, visible — by the id core derives from its machine name. Forms with fields the page is useless without (login's username and password, the site-information form's site name) additionally require those fields.
Examples
Then I should see the login form
And should see the sign-in form
Then we should see the registration form
And I should see the password reset form
Then we should see the contact us form
And I should not see the login form
Pattern
Then(/^(I |we )*should( not)* see the (login|sign-?in|registration|register|sign[ -]?up|password reset|reset password|forgot password|contact[ -]?us|contact|search|site information) form$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.