All broodies respond a bit differently. However, if she is sitting on a nest 24x7, and wants to defend that nest spot, she is likely brooding.
And yes. There are phases of brooding.
The initial phase is the egg gathering phase where the hen (and her hormones) are contemplating brooding. She will sit for a time, but may still lay eggs as she builds her clutch. She is likely to steal eggs from other nests at this time. Many hens puff at this time and peck furiously at anything that disturbs them or their nest. The size of the clutch literally presses on a spot on their sternum that helps to release brooding hormones. So once the clutch size is sufficient, the hormone level takes the hen to stage 2.
The deep phase 2 is when the hen settles onto a clutch for serious incubation. Eggs don't start growing embryos until they have been covered at 101 degrees for 24 hours. At this point, the hen often goes into a "zombie" state and does not stir from the nest but will appear to be in a trance. She may not notice things that happen near her. While still defensive of her nest, she may take a bit longer to respond. A hen in a deep brood will generally get up once a day for about 20 minutes to relieve one very large and smelly poop, eat, and drink. Game hens are known for flying off the nest for these outings by launching straight up and out squawking like crazy to draw predators away from the vacated nest to protect it. Yours may or may not do that. I have several non-game hens that do, and a number that don't.
Lock down phase is like zombie phase only the hen remains on the nest for the last 2 or 3 days straight until the chicks have all hatched or she believes the eggs are duds. Most hens will fiercely defend against being disturbed at this point. If they are, they may take any hatched chicks and abandon the unhatched eggs (so don't disturb them during this crucial time). New moms may look nervous as the hatching commotion below becomes more rambunctious. They may even jump a bit. (Another reason to make sure mom isn't disturbed).
New babe stage is when mom abandons the nest to take the hatchlings to cleaner areas and to food and water. She will guard her chicks settling periodically to warm them as they are very sleepy the first few days. Often momma hens will molt as the hormone levels change (which is a good thing as those feathers have a load of crud in them from brooding and the little dears running, and pooping, in their feathers.
Keep watch on your hen. If you want her to brood, let her have a quiet area and a clutch of eggs and see if she gets to work.
HTH
LofMc