A good approximation should be requesting /t/<id>/1.json and checking for this property:
If missing, it’s probably the about post. (There might be other reasons for not being able to delete it.)
You might be better off running a Ruby script, which will be able to do something like this way more quickly. In the Rails console, this is how you get the id of the about topic for the staff category:
Category.find_by_slug("staff").topic_id