A broody hen, when you free range your flock during the day, can sometimes appear to be lost. Not too many people live way out in the mountains, but I do, and awhile back I thought one of my favorite hens had been attacked and probably killed. I found her though, sitting on a clutch of eggs hidden by tall grass toward the edge of my property.
When this happens, it's advisable to put up a fence around her until the baby chicks have hatched. Doesn't need to be very big . . . maybe 3 square feet for standard breeds or 2 for bantams. I often drive tent stakes into the ground to keep animals out and drape a net over the enclosure. Then, put water and feed in the enclosure for her. Check on her as often as possible, but imagine a "do not disturb" sign is up.
In a case like this, you really can't tell what stage the eggs are in, and it's not advisable to pick the hen off the nest to check. You've maybe experienced their displeasure if you've ever attempted that - they can peck pretty hard, and actually injure you.
Only thing you can do is listen for peeps or watch for tiny heads popping out from beneath the mama hen. Then you'll know it's time to set out some chick starter and water for the babies. Some people will move the hen and eggs closer in to a more protected area when found, others prefer to leave her as protected as possible undisturbed. After the babies come out from under the hen, the little family can be easily moved closer to the rest of the flock, but should be kept separate from the rest for awhile just for safety. If it's really cold, you can add a heatlamp or bring the babies into a brooder.