I took the advice from another poster here on BYC and it has been the best advice anyone has given me.
To determine whether or not your hen is actually "broody" then she has to spend TWO consecutive nights on the nest, and NOT up with the others roosting on something. Sometimes they may spend one night (perhaps she has an egg that will be laid first thing in the morning). Sometimes, they may spend two non-consecutive nights for whatever reason. But unless she spends two CONSECUTIVE nights on the nest, don't count on her.
My bantam hens will sit in their nest box for HOURS before laying their egg. The large fowl will only rest there about 30 minutes or so before laying and moving on. But both of them will arrange nesting material there, fall asleep, and even stand up and rearrange the eggs every now and then, pulling them tighter underneath her. But they won't leave the nest until they have laid their egg. Once that happens, they are up and off with the flock again.