The chance it was a mistake is about 1 in ten thousand... dairy folks are very possessive of their heifers. If she were good, she'd be worth around $500 right now. You can have her rectally palpated (have it done by an experienced large animal vet) after she's about 375lbs or more. I think I've had them checked down to 350lbs, but I'm not sure on that. They'll be able to tell... most freemartins lack a complete reproductive tract, and depending on the degree of severity she may lack the entire thing - cervix, uterus, ovaries.