You will know. I know that sounds superficial, but its true. My whole pregnancy with my daughter I was so worried I wouldn't know when I was in labor. Then one morning I told my mom call the hospital tell them we're coming in. She didn't believe me, because I was acting fine and in no pain at all with contractions. When we got to the hospital I was already 6cm.
I've been having cramping for like a week and a half now, but no water breaking or anything so I'm not sure.
If contractions are inconsistent and stop or if your not having contractions at all