it's possible, but the answer may or may not be right. when my mom had my little sister, my mom asked around 15 weeks, and they told us boy. then the next one they still said boy. they always said boy, then she came and it was a girl, not a boy. I asked my mom and she recommends waiting until around 25-30 weeks, because the bigger/further along the baby is, the more likely the doctor will have a good idea and likely you will get a correct answer. there are still chances of wrong answer, but the bigger the baby is, the easier it is to tell. the only for sure way is wait until birth.
Around 16 weeks, however that is if the baby cooperates and is in good position. She / he may be hiding and ultrasound isn't 100% accurate. Most insurance companies only allow 2 ultrasounds per normal pregnancy, one at approximately 16 weeks and another at the end. Another way to verify is via amniocentesis which is done in high risk pregnancy and to verify lung development in some cases. Good luck ! Hope she cooperates! : )