Assert the site has (or has no) checked out items, read from the Global Check-in page, which lists the tables that hold locked items.
Examples
Then there should be checked out items
Then there should not be checked out items
And there should not be checked out items
Then there should be checked out items
And there should be checked out items
Pattern
Then(/^there should( not)* be checked out items$/, …)
Part of @webship-js/joomla-steps (github.com/webship/joomla-steps), a step pack for Webship-js 2.0.x.