With one simple equation you can calculate the GI from any food label. The math is simple. To assess how a packaged food will affect your blood sugar, find the carbohydrate number in the label, subtract the grams of fiber and sugar alcohols. Your result will be the grams of digestible carbohydrates
The glycemic index (GI) is a concept that ranks the glycemic potency of foods (1). It is calculated as the incremental area under the curve (iAUC) for blood glucose after consumption of a test food divided by the iAUC of a reference food containing the same amount of carbohydrate. GL is a GI-weighted measure of carbohydrate content. ... Glycemic load of a serving of food can be calculated as its carbohydrate content measured in grams (g), multiplied by the food's GI, and divided by 100. For example, watermelon has a GI of 72. The glycemic load takes into consideration both the glycemic index of a food and the amount of carbohydrate in the portion of food eaten. The glycemic load is calculated by multiplying the glycemic index value by the number of grams of carbohydrate, then dividing by 100.
General assumptions of GI can be done regarding the experience mostly as I believe. However, when following parameters of food products are known to us; it is easy to get an idea of their GI s;
1). The degree of gelatinization - easy to identify (e.g. The outer surface of the bread is lower in GI compared to the interior due to retrogradation..., so is for roti. But for foods like noodles, boiled rice - GI is higher due to high degree of gelatinization).
2). Percentage of complex carbohydrates and their sources (This can be readily understood by reading ingrediets that are in their respective order).
3). Incorporation of simple sugars. (Again can be known by food ingredients labeled).
There are some other aspects to consider too, but they can not be used at a glance to generally assume the GI.