Further to the previous answer, food labels generally give the carb and sugar content. If it says "20g carb, of which 8g sugar" what this means is that, out of the 20g, 8g will convert into blood sugar within seconds. The other 12g will convert into blood sugar but slower. Just how slow will depend on the glycemic index (GI) of the item. If it has a high GI then it will convert into BS quickly, usually in around 10 to 15 minutes. If it has a low GI then it could take up to 2 hours to convert to blood sugar.
It is because of this that foods with a low GI are generally reckoned to be better for diabetics, because your blood won't get spiked with a sudden increase in blood sugar that your body's insulin won't be able to cope with. Even better if you can try to cut right down on the carbohydrates as well as making sure that those you do eat are low GI.