A spayed dog will have a small, smooth scar on her belly where the incision was made to remove her ovaries and uterus. This scar is usually about 2-3 inches long and is located just below the belly button. In some cases, the scar may be hidden by fur, but it can usually be felt by gently running your hand over the dog's belly.
Another way to tell if a dog is spayed is to look at her vulva. In an unspayed dog, the vulva will be a small, round opening located just below the tail. In a spayed dog, the vulva will be a smooth, closed line.
Finally, you can also ask your veterinarian to confirm whether or not a dog is spayed.