I was thinking more skin color or possibly changes in the eye (if I remember right eyes get swollen when blood sugar is high) Honestly, I don't know if there are enough outwardly visible changes for this to even be possible.
But there's a lot of money out there waiting on the person who can figure out a way to use a cell phone do something like this.
Good idea, but apparently much money has been spent on it already. Excellent article on the physics of all things non-invasive (but especially glucose): http://www.mendosa.com/noninvasive_glucose.pdf
