Assert a checkbox is checked or unchecked, found either by its visible label text (label[for], a wrapping label or a sibling label all work) or by a machine handle: the DOM id Form API derives from the field name (edit-status-value), a data-drupal-selector or a name attribute.
Examples
Then the checkbox labeled "Published" should be checked
And the checkbox labeled "Sticky at top of lists" should be unchecked
Then the checkbox with the id "edit-status-value" should be checked
And the checkbox with the id "edit-sticky-value" should be unchecked
Then the checkbox labelled "Promoted to front page" should be checked
Pattern
Then(/^the checkbox (labeled|labelled|with the id) "([^"]*)" should be (checked|unchecked)$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.