The sound the hen is making here is what I'm referring to:
EDIT: this one is even better, someone just posted it on here:
Set broodies may also screech and fuss if you mess with them in the nest.
For me if they want to sit on the nest overnight, plus exhibit at least one other broody behavior (puffing up, broody ticking sounds, plucked chest feathers) I would consider them broody enough for me to break.