When mine go broody, they stay on the nest even at night, and I can pull them out in the middle of the floor and they stay fluffed out and just sit there and cluck for a while, then try to go back in.  When I reach under them for eggs, they act totally different than one that is trying to lay.  They will lift all feathers and fluff up and complain as they sit there defiantly, where the laying hen will either be silent or peck me, or jump off the nest if it's a flighty one.
I have always had a few 'Production Reds' amongst my laying hens.  I'm told these are RIR and NHR cross, but don't know for a fact.  They have always been my 'most likely to set' layers.  I have 3 of them now and I have to break up one every 3 or 4 months.  I have had one setting at the right season occasionally and brought home day-old chicks from the feed store to slip under her in the night with a red lens head lamp on, and they have been very good mothers.  Dang it's so cool to watch them prrrrr and scoot chicks back in under her!
I finally put a plastic leg band on the hen I thought was my repeat setter, but others joined the club.  I have one Red hen setting right now, and it's nearly Christmas in Oklahoma, USA, North America!  BRRRRRRRRR!  If I knew for absolutely certain that my BSL rooster was covering even my older heavy breed hens, not just the younger set of WLs he grew up with, I'd toy with the insanity of letting her try to hatch a small clutch in the dead of winter.  I don't have a place to keep them warm if it gets wicked cold, which it will do Jan and Feb here (think ice storm city), and If I set them today, they'd hatch around Jan 8th.  I'd sure have an early start on any layers, and I really want my own free range broilers for a change, but I don't want to have to create a place in the overstuffed garage!   Ugh!  Now I have to break up a hen from setting!  I have a wire small cage with wire bottom that hangs in the outer run up high and that works well for 'setters jail' as long as it's not freezing or raining, which it has been.  I can hang it inside if I have to.  Good luck with your girl and babies!