Blood sugar can be measured after fasting (not eating) for 8 hours, or as a random sample (measured any time during the day).
Normal blood sugar levels are:
In people without diabetes, then, blood sugar should not normally be higher than 200 mg/dl (11.1 mmol/L).
It's not always appropriate to assume that everything's fine if your blood sugar is within normal levels. Our bodies all respond differently. For example, have you ever noticed that you feel sleepy after a big meal? Some people are very familiar with this feeling, and it can be a result of blood sugar spiking after consuming large amounts of food.
It's always best to have a frank & open discussion with your doctor about any symptoms of diabetes you may have been experiencing, or really, anything unusual or different from normal. If you notice that you've been feeling a little "off" after eating certain foods, or at certain times of the day, keeping track of these dates / times / meals can be a helpful tool for your doctor (and for yourself too).
The first step is to talk to your doctor and get blood glucose tests done, if you haven't done so already. This will tell you whether or not you're clinically considered pre-diabetic or diabetic.
Your doctor will go over diabetes treatment options with you. Sometimes diabetes can be adequately controlled with exercise and dietary changes; sometimes medication or insulin will be needed.
Everyone also responds differently to exercise and diet. Get a good glucometer that you're comfortable with and test your blood sugar regularly. This is especially important when you're first learning how your body responds to treatment. It might take several weeks or even several months before you're able to figure out what works best for you. Things that you can change up include:
The question of "how high can blood sugar go to be normal" can certainly be looked up on a standard chart, but health care is individual - talk to your doctor to find out what your goal should be. Some people will have a harder time than others maintaining a "normal" blood glucose.