Using this step definition you can check if any form field (text field, checkbox, etc) or button is disabled:

Then the "Name" field should be disabled
  And the "Save" button should be disabled
But the "Locked" field should not be disabled

Capybara

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Webrat

Then /^"([^\"]*)" should( not)? be disabled$/ do |label, negate|
  attributes = field_labeled(label).element.attributes.keys
  attributes.send(negate ? :should_not : :should, include('disabled'))
end