Well the ultrasound tech will try really hard to see what is or is not there if you're wanting to know what you're having before you get to the delivery room. They'll push on your belly and try to move the baby, and sometimes push on both sides to make the baby move in all directions to try and see what's there (or not as the case may be!). But really, they don't HAVE to do anything. If the baby won't cooperate, the baby won't cooperate. You can always try again at the next ultrasound appointment, or at a 3D ultrasound, etc. But since there's no "medical" reason to know whether it's a girl or a boy, they won't be horribly concerned about finding out.