Most diabetics can't tell when their blood sugar is high. It's not something you can feel. (But it -does- blur your vision!)

When you eat a lot of sugar all at once it causes an insulin reaction. Your pancreas pumps out a lot of insulin to help your body burn the sugar, which it does fairly quickly, and when the sugar is gone you have all this leftover insulin so you 'crash', you feel tired and irritated, maybe even dizzy. It's a shock to your whole body. Kids can get away with it better than adults.

If you know someone who is diabetic he or she will have a blood glucose meter and will be able to test you. That is very cheap and painless, but it will give you a big clue as to whether or not you are diabetic, and if it looks like you might be, then you can go to a doctor and get better tests.

It's also possible you are 'pre-diabetic', in which case you could take the classes on diet and exercise and start changing your habits. There is not much evidence to suggest that a pre-diabetic can prevent the onset of actual diabetes through diet and exercise, but it's certainly worth a try, and anyway, if later you do get full-blown diabetes it won't hit you as hard or do as much damage.