There are several different signs that a hen might be broody. They don't all react the same way. Some look like they are in a trance on the nest. Some make a distinctive clucking sound. Some will fluff up to try to look big and imposing if you get close. Some will peck you if you stick your hand into the nest. Some will even go to the extreme of trying to flog you if you threaten their nest. Some pluck the feathers off their breast to feather the nest and bare their breast to better transfer heat to the eggs. When she takes a dump, it is usually big and stinky.
Until you are familiar with these signs, the one method I'd suggest you use to determine if a hen is broody is to see where she is at night. If she stays on her nest all night and almost all day, she is broody. A broody will usually leave her nest once, maybe twice, a day to get a little food, water, take a constitutional, and maybe a quick dust bath. But she spends the rest of the time in the nest.
Since it sounds like you are not real familiar with broody hens, I'll include a link that I think might hep you. If she is broody, you have some decisions to make.
Isolate a Broody? Thread
https://www.backyardchickens.com/forum/viewtopic.php?id=213218
I just saw Cindiloohoo's post. I would not leave real eggs in the nest until I saw she is really broody. You can use fake eggs like golf balls, wooden eggs, plastic Easter eggs, anything like that to see how she reacts, but I think leaving eggs in the coop overnight is an invitation to egg eating preditors, like rats. They are also more apt to get broken accidentally. And you need to start all the eggs at the same time. I would collect all eggs every day to help keep them cleaner until I was ready to start them.