How do you know if baby is breech?

Lainey • Due with Baby #1 Nov. 2016!
I'm 31 weeks and haven't had an ultrasound since 20 weeks. At that time the tech mentioned that the baby was breech. I know that doesn't really mean anything at that point but I don't know if/when I'll have another ultrasound and am wondering how my doctor will know if my daughter is still breech when the time comes?