As PPs said, you'll know. I've been feeling this for weeks, and it's seriously weird. Sometimes you can even feel a little head or limb through your belly, and the rounded out bump is no longer rounded out and symmetrical. It doesn't last long, but let me reiterate... it's seriously weird lol
Sometimes it does *kinda* hurt when you touch your belly mid-roll. But that's only if it's causing some extra stretching, I think. NBD.