I kinda "winged it," LOL. Since I'd had a couple of babies already, I could tell I was approximately 1 cm for a while. When I had been having contractions for a couple of hours, I checked myself, just to see if I could discern a difference, and I could -- I guessed about 4 cm with a bulging bag of waters. (That was based on the fact that I had checked myself in early labor with my second baby, just out of curiosity, and then the MW had confirmed about 4 cm a little while later.) Plus my labor pattern matched what I knew to be about 4 cm from my previous births. Then a while later, I checked again, just out of curiosity, and while I couldn't tell how dilated I was, I could tell it was more than it had been earlier, still with the bulging bag of waters. Once my water broke, I did try checking again, because with my second baby, I had been almost completely dilated before my water broke, and then when it did, my cervix closed up several cm, and it was a couple more hours before I was fully dilated, so I wanted to see if that had happened again. I couldn't tell, though, this time. However, it didn't matter -- I never had someone tell me that I was 10 cm and could push this time; I just had a couple of contractions after my water broke that ended in small pushes (not sure if they were voluntary or not), and then I felt the unmistakable feeling of the baby crowning, and he was born a couple of minutes later, less than 20 minutes after my water broke. So obviously I was complete, or at whatever point I needed to be at for him to be birthed. Something told me to push a bit with those contractions, hoping to bring the baby (who still felt really high after my water broke) down a bit, and then it all happened really fast. Listening to your instincts is a good, good thing!
(I have read that 10 cm is somewhat subjective. First of all, different people may guess 10 cm slightly differently, especially if their finger size differs. Second, I have also read that tiny/premature babies don't necessarily need a full 10 cm, and some babies need more than 10. The latter makes sense to me -- my first two babies had heads bigger than 14" and one came out posterior, so the largest diameter very well may have been bigger than 10 cm. So with my UC, I figured I'd push when I just could not help it anymore, and really, that's what happened.)
Mama to a bunch