There is currently no non-invasive way. A company called Dexcom has a sensor and transmitter that you wear that can send data to the phone and thus the watch that tracks blood glucose level every 5 minutes. I have one as I am a type 1 diabetic. Its obviously developed for diabetics more than those casually interested in blood sugar levels. Dexcom and Google are working together to develop a wearable bandage to measure levels but that is 2-3 years out.