In over 40 years, we've had a number of sets of twins, and several have been pairs of bull + heifer. We've never tried to keep many of them because of the low probability that they would be fertile, but it is possible. I've always heard the probability was closer to 5%, so I would expect somewhere in the 5-10% range.
If she's just a baby, it really isn't gonna matter for a while. I would just hope and not expect too much at this point. There are a couple of things you can do, starting now, in order to gauge your hope and expectations. Often, you can tell that freemartin heifers aren't right from an external examination. I've had freemartins that just don't look right in the vulva area, and I remember one that only had two teats, like a steer has two small teats. The external genitalia is the first test, and it gets easier to tell with a little age, so I would give her a little time and let her develop.
Be around weaning time, you might be able to tell that things don't exactly look right. Depending on how much hope you still have at that point, you can have her palpated by a vet or a repro physiologist to determine if all the parts are there. The problem with freemartins is that they don't have a complete reproductive tract. If she passes that test, then wait and see if she'll cycle. If she comes in heat, I would say she's probably ok.
In our experience, the freemartin heifers are almost always one of the best heifers in the group for some reason. We've had a couple that we've held on to for a while, hoping they might turn out to be ok. The first one we've ever had that actually turned out to BE ok is a two-year-old first-calf-heifer right now that was born about a month premature. Her twin brother was stillborn and she almost died, but looking back now, I don't think we coulda killed this baby if we had tried! Some of em just don't want to live, but this little girl never thought about giving up. She won me over when she lived through the first night, and she just thrived every day. I studied her several times before and after weaning, and I could never find anything abnormal about her development, but she still ended up in the freezer beef pen until one day my dad found her in heat. She immediately came out and went in with the replacement heifers, and she's worked out really well. She's the only one we know of that we've ever had that has turned out not to be a freemartin. But she's proof that it IS possible!
You can do a blood test. I'm not sure how soon that can be done, but if you're really anxious to find out, and you're still hopeful after a thorough examination of the external genitalia, check with GeneSeek or Igenity or one of the labs that does a lot of bovine testing. I don't remember how much it cost, but I don't think it was unreasonable. You're not gonna change whatever the situation is, so personally, I would give her a little time and see how she develops before I spent the money on a blood test. If you still have reason for hope by weaning time, it would be a good time to move forward and invest in that.
Good luck with her!
