Cows will tell you when they are about to calve. You just have to learn to understand their body language. According to research reported in the July Journal of Dairy Science, you should watch for the following signs:
Standing activity. Cows within one day of calving spent an extra two hours standing per day compared to other close-up cows (14.4 hours per day versus 12.3 hours per day).
Up and down activity. Cows within one day of calving got up and lied down 17.3 times per day compared to only 11.7 times per day for the average close-up cow.