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 :)
I had those symptoms and I could tell by looking in the mirror. My belly seemed to just get lower one day.
I've been having a lot of pain in my pelvic and my vajayjay lately. And pee constantly.
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.