Broodiness is a hormonal thing... WalkingonSunshine is right, you can't "make" one go broody, but if you have one whose hormones are kicking in and they are teetering on the edge, so to speak, they would certainly be more prone to take over a nest already full of eggs.
We've started to finally figure out some of the early signs our hens show (but all hens are different, so new broodies can still surprise us)... our hens will start getting really grouchy with flock mates when given scratch, they will 'talk' to themselves while roaming around in the yard, they tend to walk slower with their head low and sometimes even with their wings slightly puffed out so they look larger and the other birds steer clear of them. They may spend a lot of time hanging out watching another broody with her little ones and start hopping nest boxes even after you know they laid their egg for the day. Egg hoarding can also start, with a hen suddenly starting to roam off to lay somewhere hidden rather than their normal spot.... the signs vary greatly, but once the hen is broody their seem to be some constants, as WoS mentioned....
..... the broody spread... or pancake appearance.... they seem like they flatten themselves out much more than normal to cover as much nest as possible, along with that is often a very highly fluffed appearance of their feathers... they look like they just got out of a dryer after being on 'light fluff' for a few minutes!
.... plucking a broody patch.... broodies tend to pluck an area of the lower chest so they have bare skin in contact with eggs to help them track temperatures and humidity. (I think it may also help them feel the vibrations from the eggs during later stages of development)
.... broody will stay on the nest almost constantly, she may get up for short periods for a quick meal and water or dust bath but gets very, very upset if she is blocked from getting back onto her nest. If she goes to the roost at night with the rest of the flock she may still be thinking about it, but isn't ready to commit yet.
.... I call it the 'Buddha hen' look, the trance WoS mentioned, but it is very obvious once they are into their 'zone', they other look which is also frequent is the 'glare' they give anyone (human or animal) that get anywhere near them.
Let us know how she does and what the results are... now if you don't want her to be broody right now, and she is showing signs of it, I wouldn't leave eggs in the box until she returns to normal behavior, because she will continue to try to take over them any chance she gets.
