Answer:
Well first, if the mare has been with a stallion, and she has marks on her back like the hair is gone in a few spots or small cuts then there is a good chance she has been bred too. If she starts to act differently, like if she is snooty in a field with other horses makes them move she is showing more dominance then she also might be pregnant. Also if her stomach is swelling and if you wake up one morning with a baby horse next to her that's a pretty good clue.