The most used method to tell if a mare is in heat is to use a teasing stallion and tease her. You will not be able to tell from her vulva if she is in heat. A veterinarian can palpate or use ultrasound to determine the condition of the follicular and reproductive tract. She should not be bleeding from her vulva at any time. If she has a bloody discharge from her vulva, she probably has an infection and should be treated by a veterinarian. Mares do not menstruate as human females do, so there should not be blood seen. Her reaction to a stallion will help determine if she is in heat.