With my births, I knew because I was in transition and needing to push, lol! I wasn't checked prenatally, so I was well into labor, at the end when I knew I was "complete." ;-)
I guess I felt all that stuff, but didn't know precisely what it was~ I was just thinking "Big menstrual cramps, ahhh!!!"
I never got checked in the doctor's office (my preference), so the only indicator for me was going to the hospital in labor and them saying "OH MY! You're 9cm!" I never really felt it til there at the end the last couple of centimeters... I only knew then bc my body was working really hard and I felt "open" right before I needed to push.
I had braxton hicks, and was checked and membranes stripped weekly past 35 weeks. I was restless, and couldn't get comfortable at 3cm, and at 4cm, labor was definately on. lots of belly, doubled over, stabbing pain. :)