I'm always on the lookout for it (if I see bubbles, I know it's safe), since I regularly boil water in the microwave, and I've even tried to superheat water just to see what it was like, but I've never actually succeeded at it. My understanding is that it's more about whether your container has nucleation sites for bubbles to form than about the purity of the water. My pyrex measuring cup must have plenty of scratches on the inside.