The onset of labor may come as contractions that intensify over time, or rupture of water bag.
For me, my water bag broke first and I didn't felt any contractions until about 3 hours later. When my water bag broke, I could feel a pop sound coming from my tummy, then can feel abit of wetness. Went toilet to check and saw pinkish water on my panty liner. The flow will be somewhat like menses flow, it won't be like water gushing out like tap cos the baby's head is pressing down inside acting like a stopper. And yes you will need to wear a pad before you go hospital :-)