If you want to do a test that will closely mimic what the doctor does, you can use 28 jelly beans in place of the sugar syrup the doctor uses (link here: http://www.ncbi.nlm.nih.gov/pubmed/10561636 ). YOu do this in the morning after eating nothing since the night before. You take your blood glucose reading right before the test. Then you eat the jelly beans, then you start taking blood glucose readings (bgs) about once per hour or two for at least 5 hours, preferably longer. What you are looking for is to see how high your blood sugar spikes up and how fast it goes back down. You are also checking to see if the bgs go down too far many hours later. SOme people get hypoglycemia, aka too low blood sugars, long after they eat. So you want to check for that too and that means testing for a long time after the jelly beans. This will tell you what your response to carbohydrates is, ie how far up your blood goes goes and how fast it goes back down and how far down it goes. THis is what your doctor looks for. What he/she wants to know is how effective your body is at dealing with a sudden load of carbs.
Now another thing you may want to know is how your current diet is for keeping your blood glucose from NOT spiking up. Even if the bgs spike up on carbs, eating low carb can prevent spikes and the damage spikes cause to your body. If you already have good response in the glucose challenge with the jelly beans, then you are probably fine, but it's nice to know what happens when you eat your food. It's also sometimes interesting which foods do and do not spike glucose levels. So in that case, you just eat your normal food, or maybe one of your more carby meals, and then see how good or bad your blood glucose numbers are afterward. This will let you know how well you are handling your current types of food intake. People argue about how strict you should be on judging bgs and if they are good or not, but many feel it is good if your bgs do not come up more than at about 120 or 130 and then are back down to normal within 2 or 3 hours. It is also preferred if your fasting blood glucose (bg a long time after you have eaten) is below 100 but preferably more like 80 or 90. Personally, I think close to 80 or 85 is a good number. I saw research once that people with 80 or 85 as the resting bg were much less likely to later develop diabeties than people with higher resting bgs.
Also keep in mind that stress and illness can cause bg numbers to be temporarily elevated. So to get a good idea, you may need to take several readings over different days and do not bother to do it if you don't feel really healthy that day. (unless of course you suspect hypoglycemia and what to check for that)