There are a bunch of techniques to see the heating pattern of a pan and to show how poor cast iron is in that regard. I remember someone using parchment paper (

There are a bunch of techniques to see the heating pattern of a pan and to show how poor cast iron is in that regard. I remember someone using parchment paper (maybe Harold McGee?). In looking I found someone suggesting a light dusting of flour [1]. Of course, an IR camera is the most obvious. I've also noticed water boiling in unique patterns, like you mention. [1] http://cookingissues.com/2010/02/16/heavy-metal-the-science-of-cast-iron-cooking/ http://cookingissues.com/2010/02/16/heavy-metal-the-science-...