Each breed is different and each cow within a breed has its own rythym, which doesn't help much. For instance, one of my Angus cows will have a discharge for two weeks before she calves.
Heifers are a bit easier to read. If you can watch their bags daily, note sudden enlargements. When their bags swell, start watching for a discharge. When they have a swollen birth canal, really heavy discharge and start laying around alone, or head off towards the bushes, you are only a few hours off. Be prepared to help them when the time comes and alot of that could depend on what they were bred to. Usually, Maw Nature is better at this than we are and takes care of it, but on occasion they need help.