It depends on the broody Gods.... but sometime a pile of "eggs" or golf balls in a nest that she uses will flip the switch. Kind of, sort of. Just 5-6 golf balls or fake eggs.
If she does go, (this is the hardest part) WAIT for a couple of days, because sometimes, they will start to set, kind of a practice run, then leave the nest, and then a day later get serious and stick like a tick to the nest. THATS when you want to give her eggs. I get so excited, that I many times have stuck every available egg under her, and then, have her quit, to get serious 2-3 days later, and I don't have many eggs.
Nothing is more fun than a broody hen with chicks, but it is a bit precarious.
MrsK