This photograph illustrates typical behavior of a mare in heat. The chestnut mare in this photograph is presenting to the horse behind her. Even though other horses might not be stallions, a mare strongly in heat will still present. The horse in this photograph is a gelding.
How to tell that this mare is in heat? You can tell by her body stance that this mare is in heat. Standing square with her back legs wide, lifting her tail to one side, and her vulva opening and closing are all signs that a mare is in heat and could be bred. If you have read how to tell if a mare is in heat and determined that your mare is in heat, it is important to keep her away from any stallions unless you want to breed a foal.