Glycemic index measurement is done by feeding 10 (or more) human subjects the food being tested and then drawing their blood at different times after the "meal". The procedure is rather involved. Here is a quote from Glycemic Index Laboratories (the only GIF-sanctioned North American GI test lab):
"The standard protocol requires that ten normal subjects are studied on multiple occasions (maximum 3 per week) in the morning after an overnight fast. After 2 fasting blood samples are taken, subjects eat a test meal containing 50g available carbohydrate and have further blood samples taken at 15, 30, 45, 60, 90 and 120 minutes after starting to eat. Capillary blood is obtained by finger-prick and whole blood glucose determined on-site in our laboratory with an automatic analyzer using the glucose oxidase method.
Each subject conducts one trial of each test food and 3 trials of the reference food. The reference food can be anhydrous glucose or white bread, analyzed so its composition is exactly known, and baked in our test kitchens. If large numbers of foods are being tested, a reference food trial should be done for every 6-7 test foods. This ensures that there are no changes in subject's glucose responses with time."
Clearly GI testing takes time and money. In the end, a fairly reasonable question to ask is what any of this has to do with weight loss. Again, the answer is that high GI foods can (depending upon load) cause a rapid rise in blood sugar leading to a large insulin response leading to a rapid fall in blood sugar with possible associated hunger and body fat production.