Not sure if this is the right sub. But how did people find out about diabetes and what causes it. Obviously they didn't have a blood test. Did a doctor just randomly taste a patients urine?
In his Science and Civilization in China, Needham notes that two Chinese authors wrote about a disease whereupon some people would have sweet urine without any fatty flakes in it. One of them associated it with the consumption of alcohol and starchy foods. They called it tangniaobing, or literally "Sweet Urine Sickness." We now associate this disease as being diabetes. This discovery took place around 700 AD, although I do seem to recall that diabetes was initially discovered earlier in ancient Egypt.
There is a divide between the history of the understanding of the disease in Europe and North Africa and that which was found in Asia.
In the West and around the Mediterranean the amount of urine produced is the main indicator until the Renaissance, when the influence of Islamic texts adds the sweetness of the urine as a main indicator.
In the East the focus is much more on the taste and smell of the urine itself.
The Ancient Egyptian Eber's papyrus, dated to the 15th Millenium BC but probably based on collections of much older texts, describes a disease of excessive urination which very well may have been diabetes.
Much later and much further East in the 6th c. BC the Hindu physician Sushrata is describing a disease he calls 'honey urine', this also sounds very much like diabetes.
Back in the West by the 1st c. AD the Greeks had coined the term 'diabetes' which means 'a passer though a siphon [tube]' and refers to the tendency of diabetics to urinate very soon after drinking liquids.
In the 11th c. BC the two diagnostic systems; excessive urination and sweet urine are united in Islamic medicine with the physician Ibn Sina providing a comprehensive list of known symptoms from both Western and Eastern Medicine and this understanding slowly trickles though in to European medicine, although it's not until the 17th c. that Sweetness is widely accepted as a diagnostic sign in Western Europe.
http://blog.wellcomelibrary.org/2013/11/diagnosing-diabetes-a-wee-taste-of-honey/
The disease is much more prevalent in Asian populations, especially at lower body weights, so this may explain why Asian physicians had a better understanding of its characteristics as they would be more likely to encounter it.