What I learned to do is put fake eggs in your nest boxes. Just make sure you can tell the difference, cause fake eggs take a long time to cook.
You can tell when they're getting ready to lay an egg cause they act funny. Kinda like a women giving birth how she starts nesting. When she (the hen not the woman) starts sitting and picking up pieces of grass/straw and surrounding herself you'll know.
Some also make a chirping sound or screaming (hens and women). First they chirp then they scream. (hens and women). My wife screams so much I just never know if she's giving birth or not. Our third child was born in the back seat of the car. Maybe it would help if she chirped first.
You might try putting them (the hen not the women) in the nest and show them the egg. Well I suppose you could show the women too, but I don't think you want women giving birth in your nest boxes. Of course they'd have to be big nest boxes.
I had a bantam cochin sitting on a wooden goose sized egg. That's how I knew she was broody. I always wondered where those "little" wooden eggs came from. I can't wait till it hatches.