It is a myth that all hens naturally will hatch their eggs. Many NEVER go broody. The trait has been bred out of so many breeds because it interferes with egg production.
That being said, if your banty hen goes broody, you will know it! She will stay on the nest 24/7, only getting up maybe once or twice to eat, drink, poo, and MAYBE get in a quick dust bath. She'll puff up and growl at you when you go to check eggs underneath her. If she stays on the next for two consecutive nights (no going up on the roost) and displays the other behavior, then you know she's gone broody.
Good luck - but don't hold your breath. Hope your gal goes broody for ya!