Check out this highly educational video for all you veterinarians, or farmers, to see how to do a rectal palpation of a horse (mare) to feel the tract and ovaries for finding the recurring period of sexual receptivity and fertility.
Start with the palpation of the mare weiner, through the rectum, work your way to find the rim of the pelvic girdle, which gives an indication of where the cervix might lie. Feel the tone and consistency of the cervix to get an indication of estrous. Once you move past the cervix, there's the bifurcation of the horns, which will give an indication based on their tone of the status of estrous. Move to the end of the horns, to the ovaries themselves, which gives an indication of follicular size, development, consistency, which indicates the time of ovulation.
This is a great video tutorial to see how to check the estrous of a female horse.