Most form pairs during the breeding season, but in some species, such as fur seals, the gray seal (Halichoerus grypus), and elephant seals, males (bulls) take possession of harems of cows and drive rival bulls away from their territory.
Look for the crooked tail. This is probably the best ways to tell if a cow has been bred: A cow or heifer will have a crooked tail or a tail that is held out in an odd angle than usual for 24 to 48 hours after being serviced by the bull. 1  4 Edit step.
How long it takes for a cow to give birth really depends on her body condition, her experience, how big the calf is inside her, environmental conditions, and whether the calf is presenting normally or not. As a result there really is no set-in-stone time it takes for a cow to give birth. For experienced cows, it's only a matter of 5 to 6 hours until the calf arrives. For a heifer, it can be a few to a number of hours longer. See the related question below for a similar answer. If, by any chance, you are asking how long a cow is pregnant in days, the average is around 285 days long. Please see the related question on the length of gestation below for a more detailed answer. Karin L.