They will puff up, raise their neck feathers and make a growling sound if you reach to take an egg from underneath them. Their normal broody posture in the nest is not just sitting there (you know they are crouched and semi-standing when they lay an egg, right?) but they almost flatten out really low and wide.
Plus, they do not go to roost at night, but stay on the nest.
All that they said. I sometimes have hens that do some of this, including growl or peck at you if you reach under them in the nest, and walk around outside all fluffed up and making that clucking sound, yet not really be broody. The hormiones are acting up but they don't totally kick in.
If a hen stays on the nest instead of roosting in her regular spot for two consecutive nights, I give her eggs to hatch. No matter what else she is doing, she does not get eggs until she spends two consecutive nights on the nest. Some of them will fool you.
One of my girls went broody last summer. She sat in the nest box all the time. She would steal eggs from the adjacent nest and sometimes have 14 eggs under her from thievery in the morning. My daughter was worried so she would hand feed Cluckles as she sat on the eggs and gave her drinks from a ramekin. Here are 2 pictures of her:
This is her on the nest. Hopefully you can tell she's puffed out and flattened. Imagine she is growling at me:
This is her when we forced her off the nest for fresh air, think small mad tom turkey: