How does my OB know baby is head down?

I am 33+1 when I was around 30 weeks my OB was listening to my baby's heart beat and he said she was head down for sure. At my 32 weeks check up we had to do a ultrasound due to he said my uterus was measuring really big so he wanted to kinda see how big she was. Found out she is laying side ways not up and down. He said that she will turn eventually but my concern is which way she will turn. I wasn't trying to question him because he is the one with a medical degree not me. But he told me she was head down and now she is not? I wanna know what position she is in before deliver so I can kinda prepare myself for what is to come. I don't wanna go in there thinking she is head down and come to find out she is not and they have to do a emergency c section.