The way I could tell is more pressure in my pelvic area felt like my belly was sitting in my lap and I could breathe alot better.
I've been having a lot of pain in my pelvic and my vajayjay lately. And pee constantly.
I had those symptoms and I could tell by looking in the mirror. My belly seemed to just get lower one day.
For me it was sharp quick pains in my crotch that would stop me dead in my tracks LoL!! I think it started around 37 weeks. It's the babies head bumping on your cervix and making you dilate :)