Not a diabetic, but I interact with folks who are on a regular basis.

You know if you are getting into extreme territory (up to a point... eventually the confusion that comes from hypo/hyperglycemia means you won't be aware of it).

The danger of diabetes is that you can end up perpetually a little high, which is what causes all the nasty osmotic side effects (blindness, slow healing (leading to infection and amputation), etc...). You won't be able to know that without testing.

For many diabetics, the amount of insulin they inject depends on their blood sugar. There's no way you could 'sense' it to the level necessary to control dosing.

EDIT: s/deal/interact/