I did look online at the glycemic food index and got some interesting info (who knew the watermelon would be high... good thing I don't like it!)

This might be a silly question, but how do you figure out the glycemic food index for say a packaged food? Like organic canned soup or something? Is there some kind of formula? And what about low-carb pasta, isn't that supposed to be better for people with diabetes/blood sugar issues?