The traditional Chinese method is one answer. For hundreds of years the Chinese have used a technique for estimating water temperature based on visual cues in the water such as air bubble size and water convection. This technique was documented in the famous 1049 AD treatise on tea known as The Record of Tea (茶录), which was written by Cai Xiang, a Song Dynasty politician, calligrapher, and tea connoisseur. The visual cues for identifying water temperature are as follows:
|虾眼 Shrimp Eyes - 155° to 176°F (70° to 80°C)|
|蟹眼 Crab Eyes - 176° to 185°F (80° to 85°C)|
|鱼眼 Fish Eyes - 185° to 194°F (85° to 90°C)|
|珍珠绳 Rope of Pearls - 195° to 205°F (90° to 95°C )|
|騰波鼓浪 Raging Torrent - 212°F (100°C)|
 Breville, Fetco, Bonavita, et al.