Food diaries will help you appreciate how many carbohydrates are in different foods. The amount of insulin that you need to control your blood glucose levels relates to this.
The blood sugar is also affected by the type of food that you are eating. e.g. oil slows down absorption. However thiscannot be used as a way of controlling sugars due to the other harm that oil/cholesterol does.
Food diaries are also helpful in getting you to ocntrol your weight.