How do doctors know if I am in labor?

Cervical change. Labor is defined as cervical change in the presence of regular uterine contractions. Slow cervical change < 1 CM per hour is considered latent labor, while faster cervical change is considered active labor. Active labor typically onsets at about 4 CM of dilatation, but this can vary.
Please ask your doc. And please take the birthing classes as they discuss all of that, you are in labor if you are having regular contractions that cause cervical change , please call your provider if you think you are in labor.