With my first I knew when my doctor looked at me and said, "holy shit, you're having contractions 3 minuets apart". I thought I just had a backache from being gargantuan. It was the day before my due date.
With my second I knew when they were 7 minuets apart, again, the day before my due date. I recognised the back pain.
With my twins I didn't know till I was rushed in for an emergency c-section. about a week before I was told my pelvic bone was breaking because the babies were putting too much pressure on it and I hadn't gained enough fat to cushion it, so I thought that's what was hurting me and I didn't have the back pain; had a regular doctor's appointment to monitor movement. They were born exactly 17 seconds apart at 34 weeks gestation.