She will go broody first....growl at you when you disturb her when she is on the nest, flare her feathers at you at the same time, bite you when you reach under her, cluck cluck cluck all over the place when she is out of the box. She will also stop laying. When she does these, she is ready to set on eggs. Enjoy BYC!
She will stay in the nest box, may fluff up and screech at you and peck at your hand. When she is off the nest she will be fluffed up, clucky, and pass a large FOUL smelling feces. Once she has stayed on the nest for two nights, she is ready for some fertile eggs.
Hello and Welcome To BYC! there are also a couple of good articles on broody hens in the Learning Center https://www.backyardchickens.com/a/broody-hens and https://www.backyardchickens.com/a/broody-hens-verus-incubators-the-pros-and-cons
