I learned that when something is boiling, the sides of the pan will vibrate. I never knew this growing up. When I would make hard boiled eggs, I could never tell when they started to boil. I did not realize I should listen to the sounds made and it would make a different sound when it started to boil. When I attended the training center, one lady showed me that if I touched the pan handles when the water came to a boil, they would vibrate, and that's how you know for sure when it has come to a boil. It also sounds different, with practice and experience you'll get to know when something is boiling from across the room.