There are far more symptoms to being broody than just sitting on a nest and being grumpy when disturbed.
I've had my share of broodies so I've come to learn when a hen is "going broody."
Five or so days before a hen begins nest sitting, she'll begin to pull feathers from her breast bone area, producing bald strips on each side of the bone. She will also develop the "broody cluck". It goes something like, "wop, wop wop", low and fretful. She will be easily annoyed by the others and will annoy them in return. She gets more irritable by the day.
Finally, she "goes broody," hormones clicking in a single-minded compulsion to sit on the nest. If you remove her from the nest, she will make a bee-line right back to it, as if pulled back to it by a rubber band.
However, until she lays the final egg, she won't have the compulsion to nest sit, perhaps spending long periods on and off the nest. When the final egg is laid, she will glue herself to the nest and you will only be able to discourage her by brute force. She will lay no more eggs until, either she's sat the nest for 21 days and hatched a brood, or you put her through two or three days of broody-cage treatment, then she will resume laying in a week or so.