How do i test if the response was already set/send?

I have the case that a express controller action "may" send contents.

"Send" means either content was send (http 200) or the http status was set to something (http status 204 or a redirect for example)

If nothing was sent/set a default routine should send a default content.

how can i test in my default routine if the express controller action already set contents or set the status code ?