I was going to leave this in the comments for Twisted Oaks Quarter Horses, in response to her comment about signs, but figured maybe y'all want to know the "signs" too.
Yeah, her hips are sticking out like a cow. Have been for a while. Not as pronounced as some mares I've seen, but this gal is rock hard and round normally, and now is soft and flabby and, like you said, hip bones sticking out. She's been working on her bag for about 3 months (in the beginning it was barely perceptible, just kind of swollen). Vulva has been bright red most nights lately, and totally lax. Daytimes less so. Sometimes there is a tad bit of blood, which I'm told is from varicose veins breaking. Temp dropped that first day I spent the night out there, and the day after. Was hot to the touch. I don't put too much stock in what her bag is doing because I know they can really put on a bag at the last minute. But it is getting bigger and now again has some secretion, hard and yellow this time instead of hard and amber. The night I slept out there she was wobbly in her hind end, not walking well. I've seen and felt the baby moving up by her croup. Not huge jumps though, so maybe it's just carrying the violent motion into that area, rather than legs in the birth canal. She has passed through the V-shaped belly stage just before they move into position to foal. Perhaps it moved back out of position. She has not been slab-sided as of yet, but that shouldn't come until she's just about ready. She does look thinner today, but not necessarily slab-sided. Her behavior goes from totally normal and mellow to really spaced out and introspective, to obviously in pain. She is butt rubbing, but only on one side and on her tail. Sometimes she yawns a lot or stands with her lower lip hanging (pain responses). She's had contractions. When she does she gets introspective and breathes hard. Side-biting on and off.
So anyway, there aren't many signs she hasn't shown. She's really turned down the heat on a lot of them now though.
Luckily I won't have to worry about this next year, or any other year, since she won't be bred again.