When I boil water in an open pot, I can usually recognize the different temperatures by the way the water shimmers, the size of the bubbles, etc. Do that a couple times with a thermometer, and you can probably eyeball it from there.

I've heard the stages of boiling referred to as "dragon eyes", "fish eyes", etc. See e.g. http://www.veetea.com/site/articles/Water-Temperatures/ .

That said, my wife and I got an electric teakettle with a temperature setting as a wedding gift, and we love it. That's another option. (This one, or a similar model: http://www.amazon.com/Choice-SmartKettle-Cordless-4-Quart-El... )