In my experience, udder development up to calving varies a lot by breed and genetics. I have a couple cows that make up udder 3 weeks before calving, and some only seem to fill out a couple days prior to calving. But that's one thing I do browse the herd looking for. Other things are often the heifers will go from looking very fat, to not quite as wide at the belly, when viewed from behind. Often times their sides look much more flat, and sometimes the right side especially. Additionally, you may see swelling around the vulva, and the muscles immediately below the tailhead will kind of loosen up, they won't be pulled up tight. They kind of will get a sagging look/feel. In the more immediate term you might see the cow going off by itself rather than staying right with the herd, and holding it's tail outward like they sometimes do after a day of being bred with a bull.
Good luck with the calving, and since you're getting refreshed, keep the heifers close if possible where you can watch them more diligently. Sometimes the only way I know a calf was coming soon is I walk out and there's a calf sucking on a cow.