Carrying low.. how do you know if you've dropped?

S.K
I've carried my baby super low the entire pregnancy, I've only felt him/her kick or move above my belly button only a handful of times. I never did have trouble catching my breath because the baby has really never been near the upper portion of my belly. 
The doctor said at my last appointment that baby was head down already, but didn't say much else so I'm assuming that's fine. 
So my question is, how do I know when I've "dropped" if I've never really had a high up baby?.. I've heard that you normally go into labor within a month after dropping with your first baby. (I never know what to believe though) 
I'm 31+4
Thanks!