She will be sitting on eggs (or an empty nest) continuously, not getting up for more than an hour or so at a time. And she will usually be in the same nest, not switching around sitting on one then another...unless she just has not figured out how to be broody. I've got one that lays her eggs in one nest, then goes to a different nest to sit on another hen's eggs...
If she is truly dedicated she will have a stoned look on her face. Just staring off into space and not really looking at anything. Also when you pick her up she feels like a baby with a fever, warmer than usual. She may pluck her belly/chest feathers out so more body heat reaches the eggs and you move her and she really doesn't stand, she wobbles and sits back down.
It's most noticeable when she won't leave the nest to roost. Many times a hen will hang out in a nest box or the corner of the coop for no reason, but when she does it night after night with all the other symptoms then broody is a likely reason.
Also, a normally nice hen may peck the heck out of you and NOT move when you try to pull eggs out from under her. But all of mine have had that weird look of concentration on their faces. She may also talk to herself in a low way that you haven't heard before. That is mama talking to her eggs. When she is close to hatching the eggs talk back!! I think this may be an important part of going from broody "sit-sit- sit-sit" to the next stage of leave the nest and be a mama!