If you anticipate doing GI for some time, I would recommend you just buy a small guide book, or print out useful lists from the internet, and carry them around with you until you get the hang of it.
But I think there is something to do with complex carbohydrates and fiber, if it is mostly these then the food will be low GI, whereas if it's mostly simple carbs/starch then it will be high GI. If you're buying yogurt you definitely should only get the ones with no Sugar
added (and no artificial sweetener) no matter what kind of diet you eat. Sugar
is poisonous and draws nutrition from your body to digest, definitely something to avoid if you have a health condition. Also, if small or moderate amounts of Sugar
are in another food which has lots of protein or fat, then the sugar will be slower to be absorbed, which might change its GI index. There might be some way to tell, by looking at the carbs and the fat and protein, etc.... but I don't know how. That's why I recommend just getting a list and using it until you know the details for each food, at least the ones you commonly eat.
One additional note: you might want to stay and look and learn around curezone, for alternative ways to treat your condition, and also to get a broader scope of healthful eating and lifestyle.