Crashing - BTDT Moms with IBS - how did you know it was labor?

X-post from Pregnancy:

For those that have IBS and already went through labor, how did you know it was labor and not IBS stomach pains? I'm 39wks and I've been having stomach pains for the last 4 hours. I have no idea if I'm just having stomach cramps or contractions

(My mucus plug did come out though). TIA!!