You'll know. Trust me.
When your contractions are strong enough that you can't talk through them... it's for real. They take your breath away. Sometimes your water will break first (mine did) and that's certainly enough information to want to call your provider and talk with them about their protocol for what to do next, especially if it's preterm. But active labor... I don't think there's any way to mistake it for anything else.
If u start having menstrual like cramps then ur probably beginning labor. It won't be sharp knife stabbing pain at first but the cramps will come and go and get closer together and stronger and lasting longer over the course of a few hours, when they get to about 5mins apart then u might want to go to the hospital. If ur water breaks and u haven't started any menstrual like cramping yet, go to the hospital.. otherwise labor may play out like in first scenario. Goodluck!
^^ u might also feel back pain and alot of pressure in pelvis
Sharp pain that would come and go for about 5 mins then come back again. The contractions. Or water break