Indeed, a hen can be showing signs of being broody for several weeks before she actually throws the switch to full broodiness.
The first sign of a hen going broody is the broody cluck. No hen utters this particular vocal unless her hormones are beginning to transition. It's a very low, quiet, persistent, fussy "popping". At first, this may be interspersed with her normal vocals. This won't happen until a hen has laid about a month to six weeks worth of eggs. Each hen that tends to broodiness has an inner timer that determines how many days of laying before she will go broody. I have a broody hen that I can pinpoint the very day she will go broody from her past pattern of laying. It's the same each time. But not every hen will go broody. It depends on genetics, and some simply don't have the broody gene.
The next sign is irritability. She may punctuate the "popping" with a fierce, frustrating scream. At this point, she is beginning to annoy the other flock members who may give her a peck as she wanders restlessly around doing her "popping" noises.
She'll still be laying eggs, and you may see an accumulation of loose keel feathers in the nest with her egg, and you'll notice she's going bald along the keel bone.
As she nears the point where she will stick to the nest, she will ramp up her "popping" and the scream will be accompanied by a fluffing out and fierce shaking of her feathers.
This buildup can take anywhere from one week to three, even four weeks. Then she'll lay her final egg, glue herself to a nest, and she'll fiercely and stubbornly refuse to leave it, puffing herself up and flattening herself, even biting your hand if you try to touch her.
When you know the signs, there can be no doubt you have a broody.