My rule for determining if they are broody is where they spend the night. If a hen stays on her nest two nights in a row, then I consider her broody and worthy of eggs. Otherwise, they are just messing with me.
If she is broody, you have a couple of basic options. You can leave her where she is. Hens have been hatching and raising chicks with the flock for thousands of years. Sometimes there are problems, but sometimes there are problems if you try to move one and isolate her. You need to collect the eggs every day until you have all you want her to hatch, then give them all to her at the same time. If you collect them once a day, before nightfall, they are good to eat.
If you elect to leave her, you need to mark each egg so you know which eggs belong there. I use a Sharpie and just make two circles around the egg so I can instantly see which one belongs. Then every day, you need to look under the hen and remove all the eggs that don't belong. Remember, they are still good to eat. If you don't remove them, two bad things might happen. If she collects enough eggs that she cannot cover them all, the eggs that get pushed out can cool off enough that the chick insde can die. Then, that egg gets pushed back under her and another one gets pushed out to die. You often get very poor hatches it she cannot cover them all.
The other problem is that it takes about 21 days for the egg to develop and hatch. If eggs are added late, they don't get the full 21 days, so a developing egg can be abandoned when she takes her living already hatched chicks off the nest. Gloves and long sleeve shirts or jackets can make it easier to look under a hen that defends her nest. You can either raise her up to look under her or, probably easier, remove her from the nest and put her down. She may hop right back on or take advantage of the opportunity to eat, drink, and poop before returning to the nest.
The other basic possibility is to isolate her from the flock so no other hen can get to her nest to lay an egg. We all have different set-ups and circumstances, but what you are looking for is a place you can lock her into so she cannot go back to her old nest. She needs a nest, food and water, and room to get off the nest to go poop. It needs to be predator proof. You can totally move her to a different area, fix something in the coop, or even build an enclosure around her current nest.
The big risk if you move her is that she might not be broody any more. If you move her, it is best to move her at night using as little light and commotion as possible. I find it useful if the new nest is kind of dark, or at least not in the direct light. Id seems to help if you can lock her in a dark nest the day after you move her. That may sound cruel, but its not. A broody normally stays on the nest practically all day anyway. The dark seems to help comfort and calm her.
If you move her, don't immediately give her the eggs you want her to hatch, but give her some fake eggs of some "sacrificial" eggs to see if she will accept her new quarters. If the eggs are not that valuable, you can give them to her, just realize you might lose them if she decides to not accept her new location. Most of the time you can successfully move a broody, but there are no guarantees.
Hope this helps a bit. Good luck!!!
