However, if you end up with a male/female pair, and you're a glass-half-full sort of person, there are ways to tell if your female is a heifer or a freemartin.
Touching placentas will often form into one, ensuring a freemartin. If there are two distinct placentas, the female calf may well be fertile.
Feel around for an extra opening or protrusions; both indicate a freemartin.
Due to freemartins, the practice of purchasing or retaining the female half of twins for breeding purposes should be avoided.
Since a freemartin is outwardly female, about the only accurate way to determine this status when purchasing calves is an examination by a vet.