You're asking when you will know a cows in heat, right?
From what I have seen, if the cow is in a pen, corral or pasture with either other cows, bulls or steers. The other animals with her will start sniffing around her back end. They will follow her around and will try to mount her. If she is trying to get away and not standing still, she may be coming or even going out of heat.
If she stands still and doesn't try to walk away when another animal mounts her she is in standing heat. I have also seen cows sort of squat down when they are standing heat too.
you can also just stick a heat detector patch on her back.
Shorthorn 96, wish you could have been on our farm yesterday. We had a cow that it seemed like everybody took a turn mounting, and she mounted one or two of them in return. It got to be funny because she looked like the pied piper the way they were all following her around.
Sorry to say though that she will be on the next trailer out of here. She was late calving and we kept the bull in with the cows an extra month for her. She obviously didn't catch so bye-bye.
If your cow isn't in a herd, just out of the ordinary behavior can be an indication of heat. Is she usually friendly but suddenly stand off-ish? Is she really lazy and suddenly pacing the fence? Little changes can be a sign, and with heifers it can be harder to tell.
If there's a bull around, he'll know for sure!
heat detector patches give you a general idea but are only an aid.you can tell when a heifer is coming into heat the night before (usually) not always but she'll start acting strange,walking the fence and bawlin alot its obvious.i used the patches,but the only way to really tell is to check them often,in the morning and evening not when its feed time,,its easy to tell.if their in,you'll know
Heat signs in cows can vary greatly, to no outward signs and for others it can be obvious. Last year our heifer showed us no signs at all besides having a small amount of discharge, this year after her first calf, she has been standing at the fence just bellowing all day when she is in heat.