As McGee points out, a gently bubbling pot of boiling water is only slightly lower in temperature than a rolling boil. As long as you see a constant stream of large bubbles, you’re good to go.

Quirky's Boil Buoy is a floating chime that lets you know when a pot of water boils. It also has a pun in the name which only really works if you speak English with an English accent. "Buoy" is ...