somewhat of a newbie but did watch my last broody hatch her eggs .. my experience is that she will sit on those eggs until SHE thinks the last viable one has hatched ... during those last three days she was rumbling and peeping at the eggs, and I think she waited until she had hatched any that had movement or peeping, before getting up off the nest to herd the hatched chicks around
the early-to-hatch chicks just popped in and out from under her; I'd scattered chick-starter in front as soon as I noticed the first chick ... chicks would peck-and-run back under the warmth
of her 19 eggs ... eight hatched, one was obviously a dud (lightweight so I opened it, just a little "stuff" stuck to one inner side, but otherwise empty), the other ten didn't peep or move when I checked them, and most of them "sloshed" ... no chicks in them ...
yeah, I'd candle them by Day 14  (didn't know this myself then) and discard any obviously infertile ones ... that way the broody can concentrate on the "good" ones
(she wouldn't have been setting on so many if I'd spotted the nest earlier ... one of the hazards of free-ranging ... )